code: func RequestHeaders(c context.Context) (*taskqueue.RequestHeaders, error) {
  	if hdr, ok := c.Value(&requestHeadersKey).(*taskqueue.RequestHeaders); ok {
  		return hdr, nil
  	}
  	return nil, errOutsideHandler
  }
docs: // RequestHeaders returns the special task-queue HTTP request headers for
  // the current task handler.
  //
  // Returns an error if called from outside of a task handler.