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

binding/form_mapping.go

@@ -7,6 +7,7 @@ package binding
 import (
 	"errors"
 	"fmt"
+	"net/textproto"
 	"reflect"
 	"strconv"
 	"strings"
@@ -106,6 +107,7 @@ func mapping(value reflect.Value, field reflect.StructField, setter setter, tag
 
 type setOptions struct {
 	isDefaultExists bool
+	isHeader        bool
 	defaultValue    string
 }
 
@@ -127,6 +129,7 @@ func tryToSetValue(value reflect.Value, field reflect.StructField, setter setter
 	}
 
 	var opt string
+	setOpt.isHeader = tag == "header"
 	for len(opts) > 0 {
 		opt, opts = head(opts, ",")
 
@@ -140,7 +143,17 @@ func tryToSetValue(value reflect.Value, field reflect.StructField, setter setter
 }
 
 func setByForm(value reflect.Value, field reflect.StructField, form map[string][]string, tagValue string, opt setOptions) (isSetted bool, err error) {
-	vs, ok := form[tagValue]
+	var (
+		vs []string
+		ok bool
+	)
+
+	if opt.isHeader {
+		vs, ok = form[textproto.CanonicalMIMEHeaderKey(tagValue)]
+	} else {
+		vs, ok = form[tagValue]
+	}
+
 	if !ok && !opt.isDefaultExists {
 		return false, nil
 	}

context_test.go

@@ -1443,15 +1443,18 @@ func TestContextBindHeader(t *testing.T) {
 	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())
 }
 
@@ -1569,15 +1572,18 @@ func TestContextShouldBindHeader(t *testing.T) {
 	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())
 }