code: function(url, params, onsuccess, onfail) {
      var t = _typeof(params), cn = 'Content-Type', cv = 'application/';
  
      if (t == 'object') {
        params = _params_2_url(params);
        setRequestHeader(cn, cv + 'x-www-form-urlencoded');
      }
      else if (t == 'formdata') {} // for formdata type.
      else setRequestHeader(cn, cv + 'json');
      // if is string / formdata
      return request('POST', url, params, 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;