code: function Kernel(options, requester) {
      var headers = options.headers;
      // POST请求添加默认的`Content-Type`
      if (options.method === METHOD.POST
          && !util.findHeader(headers, 'Content-Type')
      ) {
          headers['Content-Type'] = 'application/x-www-form-urlencoded';
      }
  
      var data = options.data;
      if (!util.isString(data) && options.stringify !== false) {
          data = util.stringify(data);
      }
  
      log(requester, 'info', 'request %s %s %s %s', options.method, options.url, JSON.stringify(headers), data);
  
      var agents;
      var context = requester.context;
      var agentOptions = context.data.agent;
      if (agentOptions) {
          agents = createAgents(agentOptions);
      }
  
      var xhr = request(
          {
              url: options.url,
              method: options.method,
              headers: headers,
              body: data,
              timeout: options.timeout
          },
          function (error, res, data) {
              if (!error) {
                  // 与client端兼容
                  var status = res.status = res.statusCode;
                  var level = status >= 400 ? 'error' : 'info';
                  log(requester, level, 'response %s %s\n%s', status, JSON.stringify(res.headers), data);
              }
              else {
                  log(requester, 'error', error.stack);
                  if (error.code) {
                      error = error.code;
                  }
              }
              requester.handle(error, res, data);
          },
          agents
      );
  
      return xhr;
  }
docs: 请求核心
  
  @constructor
  @param {Object} options 请求参数
  @param {Requester} requester 请求对象