code: function(url, params, onsuccess, onfail) {
      // url中包含 get 参数和 params 合并
      if (_typeof(params) == 'object') params = _params_2_url(params);
      else params = '';
  
      if (params) url = url + '?' + params;
      return request('GET', url, '', onsuccess, onfail);
    }
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;