support bind http header param gin-gonic#1956 (gin-gonic#1957)

* support bind http header param gin-gonic#1956

update gin-gonic#1956
```
package main

import (
	"fmt"
	"github.com/gin-gonic/gin"
)

type testHeader struct {
	Rate   int    `header:"Rate"`
	Domain string `header:"Domain"`
}

func main() {
	r := gin.Default()
	r.GET("/", func(c *gin.Context) {
		h := testHeader{}

		if err := c.ShouldBindHeader(&h); err != nil {
			c.JSON(200, err)
		}

		fmt.Printf("%#v\n", h)
		c.JSON(200, gin.H{"Rate": h.Rate, "Domain": h.Domain})
	})

	r.Run()

// client
// curl -H "rate:300" -H "domain:music" 127.0.0.1:8080/
// output
// {"Domain":"music","Rate":300}
}
```

* add unit test

* Modify the code to get the http header

When the http header is obtained in the standard library,
the key value will be modified by the CanonicalMIMEHeaderKey function,
and finally the value of the http header will be obtained from the map.
As follows.
```go
func (h MIMEHeader) Get(key string) string {
        // ...
         v := h[CanonicalMIMEHeaderKey(key)]
        // ...
}
```

This pr also follows this modification

* Thanks to vkd for suggestions, modifying code

* Increase test coverage

env GOPATH=`pwd` go test github.com/gin-gonic/gin/binding -coverprofile=cover.prof
ok  	github.com/gin-gonic/gin/binding	0.015s	coverage: 100.0% of statements

* Rollback check code

* add use case to README.md

README.md

@@ -40,6 +40,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [Only Bind Query String](#only-bind-query-string)
     - [Bind Query String or Post Data](#bind-query-string-or-post-data)
     - [Bind Uri](#bind-uri)
+    - [Bind Header](#bind-header)
     - [Bind HTML checkboxes](#bind-html-checkboxes)
     - [Multipart/Urlencoded binding](#multiparturlencoded-binding)
     - [XML, JSON, YAML and ProtoBuf rendering](#xml-json-yaml-and-protobuf-rendering)
@@ -910,6 +911,43 @@ $ curl -v localhost:8088/thinkerou/987fbc97-4bed-5078-9f07-9141ba07c9f3
 $ curl -v localhost:8088/thinkerou/not-uuid
 ```
 
+### Bind Header
+
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/gin-gonic/gin"
+)
+
+type testHeader struct {
+	Rate   int    `header:"Rate"`
+	Domain string `header:"Domain"`
+}
+
+func main() {
+	r := gin.Default()
+	r.GET("/", func(c *gin.Context) {
+		h := testHeader{}
+
+		if err := c.ShouldBindHeader(&h); err != nil {
+			c.JSON(200, err)
+		}
+
+		fmt.Printf("%#v\n", h)
+		c.JSON(200, gin.H{"Rate": h.Rate, "Domain": h.Domain})
+	})
+
+	r.Run()
+
+// client
+// curl -H "rate:300" -H "domain:music" 127.0.0.1:8080/
+// output
+// {"Domain":"music","Rate":300}
+}
+```
+
 ### Bind HTML checkboxes
 
 See the [detail information](https://github.com/gin-gonic/gin/issues/129#issuecomment-124260092)

binding/binding.go

@@ -78,6 +78,7 @@ var (
 	MsgPack       = msgpackBinding{}
 	YAML          = yamlBinding{}
 	Uri           = uriBinding{}
+	Header        = headerBinding{}
 )
 
 // Default returns the appropriate Binding instance based on the HTTP method

binding/binding_test.go

@@ -667,6 +667,31 @@ func TestExistsFails(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestHeaderBinding(t *testing.T) {
+	h := Header
+	assert.Equal(t, "header", h.Name())
+
+	type tHeader struct {
+		Limit int `header:"limit"`
+	}
+
+	var theader tHeader
+	req := requestWithBody("GET", "/", "")
+	req.Header.Add("limit", "1000")
+	assert.NoError(t, h.Bind(req, &theader))
+	assert.Equal(t, 1000, theader.Limit)
+
+	req = requestWithBody("GET", "/", "")
+	req.Header.Add("fail", `{fail:fail}`)
+
+	type failStruct struct {
+		Fail map[string]interface{} `header:"fail"`
+	}
+
+	err := h.Bind(req, &failStruct{})
+	assert.Error(t, err)
+}
+
 func TestUriBinding(t *testing.T) {
 	b := Uri
 	assert.Equal(t, "uri", b.Name())

binding/header.go

@@ -0,0 +1,34 @@
+package binding
+
+import (
+	"net/http"
+	"net/textproto"
+	"reflect"
+)
+
+type headerBinding struct{}
+
+func (headerBinding) Name() string {
+	return "header"
+}
+
+func (headerBinding) Bind(req *http.Request, obj interface{}) error {
+
+	if err := mapHeader(obj, req.Header); err != nil {
+		return err
+	}
+
+	return validate(obj)
+}
+
+func mapHeader(ptr interface{}, h map[string][]string) error {
+	return mappingByPtr(ptr, headerSource(h), "header")
+}
+
+type headerSource map[string][]string
+
+var _ setter = headerSource(nil)
+
+func (hs headerSource) TrySet(value reflect.Value, field reflect.StructField, tagValue string, opt setOptions) (isSetted bool, err error) {
+	return setByForm(value, field, hs, textproto.CanonicalMIMEHeaderKey(tagValue), opt)
+}

context.go

@@ -583,6 +583,11 @@ func (c *Context) BindYAML(obj interface{}) error {
 	return c.MustBindWith(obj, binding.YAML)
 }
 
+// BindHeader is a shortcut for c.MustBindWith(obj, binding.Header).
+func (c *Context) BindHeader(obj interface{}) error {
+	return c.MustBindWith(obj, binding.Header)
+}
+
 // BindUri binds the passed struct pointer using binding.Uri.
 // It will abort the request with HTTP 400 if any error occurs.
 func (c *Context) BindUri(obj interface{}) error {
@@ -637,6 +642,11 @@ func (c *Context) ShouldBindYAML(obj interface{}) error {
 	return c.ShouldBindWith(obj, binding.YAML)
 }
 
+// ShouldBindHeader is a shortcut for c.ShouldBindWith(obj, binding.Header).
+func (c *Context) ShouldBindHeader(obj interface{}) error {
+	return c.ShouldBindWith(obj, binding.Header)
+}
+
 // ShouldBindUri binds the passed struct pointer using the specified binding engine.
 func (c *Context) ShouldBindUri(obj interface{}) error {
 	m := make(map[string][]string)

context_test.go

@@ -1436,6 +1436,28 @@ func TestContextBindWithXML(t *testing.T) {
 	assert.Equal(t, 0, w.Body.Len())
 }
 
+func TestContextBindHeader(t *testing.T) {
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+
+	c.Request, _ = http.NewRequest("POST", "/", nil)
+	c.Request.Header.Add("rate", "8000")
+	c.Request.Header.Add("domain", "music")
+	c.Request.Header.Add("limit", "1000")
+
+	var testHeader struct {
+		Rate   int    `header:"Rate"`
+		Domain string `header:"Domain"`
+		Limit  int    `header:"limit"`
+	}
+
+	assert.NoError(t, c.BindHeader(&testHeader))
+	assert.Equal(t, 8000, testHeader.Rate)
+	assert.Equal(t, "music", testHeader.Domain)
+	assert.Equal(t, 1000, testHeader.Limit)
+	assert.Equal(t, 0, w.Body.Len())
+}
+
 func TestContextBindWithQuery(t *testing.T) {
 	w := httptest.NewRecorder()
 	c, _ := CreateTestContext(w)
@@ -1543,6 +1565,28 @@ func TestContextShouldBindWithXML(t *testing.T) {
 	assert.Equal(t, 0, w.Body.Len())
 }
 
+func TestContextShouldBindHeader(t *testing.T) {
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+
+	c.Request, _ = http.NewRequest("POST", "/", nil)
+	c.Request.Header.Add("rate", "8000")
+	c.Request.Header.Add("domain", "music")
+	c.Request.Header.Add("limit", "1000")
+
+	var testHeader struct {
+		Rate   int    `header:"Rate"`
+		Domain string `header:"Domain"`
+		Limit  int    `header:"limit"`
+	}
+
+	assert.NoError(t, c.ShouldBindHeader(&testHeader))
+	assert.Equal(t, 8000, testHeader.Rate)
+	assert.Equal(t, "music", testHeader.Domain)
+	assert.Equal(t, 1000, testHeader.Limit)
+	assert.Equal(t, 0, w.Body.Len())
+}
+
 func TestContextShouldBindWithQuery(t *testing.T) {
 	w := httptest.NewRecorder()
 	c, _ := CreateTestContext(w)