serialize bugfix

fielder.go

@@ -155,7 +155,9 @@ func fieldVal(val reflect.Value, fieldName string, files RequestFiles, names ...
 			); err != nil {
 				return false
 			}
-			jm[rField.Name] = fVal.Interface()
+			if fVal.IsValid() && !fVal.IsZero() {
+				jm[rField.Name] = fVal.Interface()
+			}
 			return true
 		}
 		for _, name := range names {