retry to Unmarshal form value with quoted

and add test

binding/form_mapping.go

@@ -209,7 +209,13 @@ func setWithProperType(val string, value reflect.Value, field reflect.StructFiel
 		case time.Time:
 			return setTimeField(val, field, value)
 		}
-		return json.Unmarshal(bytesconv.StringToBytes(val), value.Addr().Interface())
+		err1 := json.Unmarshal(bytesconv.StringToBytes(val), value.Addr().Interface())
+		if err1 != nil {
+			if err2 := json.Unmarshal(bytesconv.StringToBytes(strconv.Quote(val)), value.Addr().Interface()); err2 != nil {
+				return fmt.Errorf("%v;tryping to parse with quoted json string but still failed: %v", err1, err2)
+			}
+		}
+		return nil
 	case reflect.Map:
 		return json.Unmarshal(bytesconv.StringToBytes(val), value.Addr().Interface())
 	default:

binding/form_mapping_test.go

@@ -5,6 +5,7 @@
 package binding
 
 import (
+	"encoding/json"
 	"reflect"
 	"testing"
 	"time"
@@ -190,6 +191,46 @@ func TestMappingTime(t *testing.T) {
 	assert.Error(t, err)
 }
 
+type CustomDateTime time.Time
+
+const CustomDateTimeFormat = "2006/01/02 15:04:05"
+
+func (t CustomDateTime) String() string {
+	return time.Time(t).Format(CustomDateTimeFormat)
+}
+
+func (t *CustomDateTime) UnmarshalJSON(data []byte) error {
+	var value string
+	err := json.Unmarshal(data, &value)
+	if err != nil {
+		return err
+	}
+	parseTime, err := time.Parse(CustomDateTimeFormat, value)
+	if err != nil {
+		return err
+	}
+	*t = CustomDateTime(parseTime)
+	return nil
+}
+
+func TestMappingCustomStructFromJsonString(t *testing.T) {
+	var s struct {
+		Time CustomDateTime
+	}
+	value := "2020/09/23 13:20:49"
+
+	// ok
+	err := mappingByPtr(&s, formSource{"Time": {value}}, "form")
+	assert.NoError(t, err)
+	expectedTime, err := time.Parse(CustomDateTimeFormat, value)
+	assert.NoError(t, err)
+	assert.Equal(t, CustomDateTime(expectedTime), s.Time)
+
+	// parse failed
+	err = mappingByPtr(&s, formSource{"Time": {"20200/09/23 13:20:49"}}, "form")
+	assert.Error(t, err)
+}
+
 func TestMappingTimeDuration(t *testing.T) {
 	var s struct {
 		D time.Duration