perf: optimize the http.ContentType method

[1911860538]

Improve performance and readability, concatenating strings with "+" instead of strings.Join.

Below is my local test code:

http.go

package httputil

import (
	"strings"
)

const (
	baseContentType = "application"
)

// ContentType returns the content-type with base prefix.
func ContentType(subtype string) string {
	return strings.Join([]string{baseContentType, subtype}, "/")
}

func NewContentType(subtype string) string {
	return baseContentType + "/" + subtype
}

http_test.go

package httputil

import (
	"testing"
)

func TestContentTypeSame(t *testing.T) {
	subtypes := [...]string{"x-www-form-urlencoded", "json"}
	for _, subtype := range subtypes {
		contentType := ContentType(subtype)
		newContentType := NewContentType(subtype)
		if contentType != newContentType {
			t.Fatalf("got different content type, expected %s got %s", contentType, newContentType)
		}
	}
}

func BenchmarkContentType(b *testing.B) {
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		ContentType("json")
	}
}

func BenchmarkNewContentType(b *testing.B) {
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		NewContentType("json")
	}
}

test output

=== RUN   TestContentTypeSame
--- PASS: TestContentTypeSame (0.00s)
PASS

Process finished with the exit code 0

bench output

goos: darwin
goarch: arm64
pkg: github.com/go-kratos/kratos/v2/internal/httputil
BenchmarkContentType
BenchmarkContentType-11       	57805406	        20.32 ns/op	      16 B/op	       1 allocs/op
BenchmarkNewContentType
BenchmarkNewContentType-11    	100000000	        10.02 ns/op	       0 B/op	       0 allocs/op
PASS

Process finished with the exit code 0