binding: support []byte

* form1.go
```golang
package main

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

type testForm struct {
	Mode   string `form:"mode"`
	Text   string `form:"text"`
	Voice  []byte `form:"voice"`
	Voice2 []byte `form:"voice2"`
}

func main() {
	router := gin.Default()
	router.POST("/test.form", func(c *gin.Context) {

		t2 := testForm{}
		if err := c.ShouldBind(&t2); err != nil {
			fmt.Printf("err = %s:%v\n", err, t2)
			c.JSON(500, gin.H{"err": err.Error()})
			return
		}
		c.JSON(200, t2)
	})

	router.Run()
}

//client
/*
curl -F mode=A -F text="test" -F voice=@form1.go -F voice2="voice" 127.0.0.1:8080/test.form|jq
{
  "Mode": "A",
  "Text": "test",
  "Voice": "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",
  "Voice2": "dm9pY2U="
}
*/

```

binding/form_mapping.go

@@ -146,6 +146,15 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 		if !ok {
 			vs = []string{opt.defaultValue}
 		}
+
+		switch value.Interface().(type) {
+		case []byte:
+			if len(vs) > 0 {
+				value.Set(reflect.ValueOf([]byte(vs[0])))
+				return true, nil
+			}
+		}
+
 		return true, setSlice(vs, value, field)
 	case reflect.Array:
 		if !ok {

binding/multipart_form_mapping.go

@@ -6,6 +6,7 @@ package binding
 
 import (
 	"errors"
+	"io/ioutil"
 	"mime/multipart"
 	"net/http"
 	"reflect"
@@ -39,6 +40,22 @@ func setByMultipartFormFile(value reflect.Value, field reflect.StructField, file
 			return true, nil
 		}
 	case reflect.Slice:
+		switch value.Interface().(type) {
+		case []byte:
+			fd, err := files[0].Open()
+			if err != nil {
+				return false, err
+			}
+			defer fd.Close()
+			c, err := ioutil.ReadAll(fd)
+			if err != nil {
+				return false, err
+			}
+
+			value.Set(reflect.ValueOf(c))
+			return true, nil
+		}
+
 		slice := reflect.MakeSlice(value.Type(), len(files), len(files))
 		isSetted, err = setArrayOfMultipartFormFiles(slice, field, files)
 		if err != nil || !isSetted {