support default value

[yfming93]

eg:

@JsonSerializable()
class SdMsgList {

String notification_id = '';
String id = '';
String is_read = '';
String read_at = '';
String user_type = '';
String unit_id = '';
String user_id = '';
String type = '';
String category = '';
String relation_data_id = '';
String relation_data_code = '';
String title = '';
String content = '';
List<String> attach = [];
// List<SomeRootEntityListTarget_info> target_info;
String read_status = '';
String remark = '';
String created_at = '';
String type_text = '';
String category_text = '';
String read_status_text = '';

SdMsgList();

factory SdMsgList.fromJson(Map<String, dynamic> json) => $SdMsgListFromJson(json);

Map<String, dynamic> toJson() => $SdMsgListToJson(this);

@OverRide
String toString() {
return jsonEncode(this);
}
}