code: function getResponseData(xhr) {
          var res;
          if (xhr.responseType
              && xhr.responseType !== RESPONSE_TYPE.TEXT
          ) {
              res = xhr.response;
          }
          else {
              res = xhr.responseText;
          }
  
          return res;
      }
docs: 根据responseType获取返回内容
  
  @inner
  @param {Object} xhr 请求对象
  @return {*}