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}
}
```

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/form_mapping.go

@@ -21,6 +21,10 @@ func mapUri(ptr interface{}, m map[string][]string) error {
 	return mapFormByTag(ptr, m, "uri")
 }
 
+func mapHeader(ptr interface{}, m map[string][]string) error {
+	return mapFormByTag(ptr, m, "header")
+}
+
 func mapForm(ptr interface{}, form map[string][]string) error {
 	return mapFormByTag(ptr, form, "form")
 }

binding/header.go

@@ -0,0 +1,18 @@
+package binding
+
+import "net/http"
+
+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)
+}

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)