feat(binding): add UseNumber() in gin.Context.BindJSON() (#997) close #368

* resolve #368 add option to UseNumber() in gin.Context.BindJSON()

* add test

binding/json.go

@@ -10,7 +10,10 @@ import (
 	"github.com/json-iterator/go"
 )
 
-var json = jsoniter.ConfigCompatibleWithStandardLibrary
+var (
+	json                   = jsoniter.ConfigCompatibleWithStandardLibrary
+	EnableDecoderUseNumber = false
+)
 
 type jsonBinding struct{}
 
@@ -20,6 +23,9 @@ func (jsonBinding) Name() string {
 
 func (jsonBinding) Bind(req *http.Request, obj interface{}) error {
 	decoder := json.NewDecoder(req.Body)
+	if EnableDecoderUseNumber {
+		decoder.UseNumber()
+	}
 	if err := decoder.Decode(obj); err != nil {
 		return err
 	}