code: function(params) {
      var encode = encodeURIComponent,
        p = [],
        key;
      for (key in params) p.push(encode(key) + '=' + encode(params[key]));
  
      return p.join('&');
    }
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;