message: Headers are an empty array by default, updated doc.
diff: /**
  * Return the response. This blocks until a response is available.
  *
  - * @param {{result: function({=response: {=body: string, =content: Uint8Array}, =isTimeout: boolean, =isError: boolean}), error: function(Error)}}
  + * @param {{result: function({=response: {=body: string, =content: Uint8Array, =headers: ?Array<string>}, =isTimeout: boolean, =isError: boolean}), error: function(Error)}}
  * callback the callback will receive the HTTP response or
  * undefined if the HTTP transaction was aborted and notified of
  * any thrown exceptions.
  _timedout: { value: false, writable: true, enumerable: false, configurable: false },
  _aborted: { value: false, writable: true, enumerable: false, configurable: false },
  _json: { value: undefined, writable: true, enumerable: false, configurable: false },
  - _headers: { value: undefined, writable: true, enumerable: false, configurable: false }
  + _headers: { value: [], writable: true, enumerable: false, configurable: false }
  };
  Object.defineProperties(this, props);
  },
  },
  /**
  - * <p>Returns an array of HTTP response headers, or undefined.</p>
  + * <p>Returns an array of HTTP response headers.</p>
  *
  - * @return {?Array{string}} an array of HTTP response headers or undefined.
  + * @return {Array{string}} an array of HTTP response headers.
  */
  getHttpHeaders: function getHttpHeaders() {
  return this._headers;
  