Dio添加Cookie

在使用Options添加headers时，Map没有定义内部类型：

    Dio dio = new Dio();
    Map headers = new Map();
    headers['Cookie'] = cookie;
    Options options = new Options(
      headers:headers
    );
    Future response;
    response = dio.get(url,options: options);

结果请求一直失败，添加上Map内部类型的定义，请求成功。

    Dio dio = new Dio();
    Map<String, dynamic> headers = new Map();
    headers['Cookie'] = cookie;
    Options options = new Options(
      headers:headers
    );
    Future response;
    response = dio.get(url,options: options);