code: function() {
      _xhr.open(_method, _url, _async);
      // 设置 header 内容
      for (var key in _headers) _xhr.setRequestHeader(key, _headers[key]);
      _xhr.onreadystatechange = __on_request_callback;
      _xhr.send(_body);
      return _this;
    }
docs: Result: the function to get `Result` instance. The request result object.
  - xhrObj: the xhr obj which can get the xhr result.
  
  How to use it? if `r` is the instance of `Result`.
  var r_json = r.json();
  var status = r.status_code;
  var headers = r.headers;