delay/delay.go
func RequestHeaders(c context.Context) (*taskqueue.RequestHeaders, error) {
	if ret, ok := c.Value(headersContextKey).(*taskqueue.RequestHeaders); ok {
		return ret, nil
	}
	return nil, errOutsideDelayFunc
}
func RequestHeaders(c context.Context) (*taskqueue.RequestHeaders, error) {
	if ret, ok := c.Value(headersContextKey).(*taskqueue.RequestHeaders); ok {
		return ret, nil
	}
	return nil, errOutsideDelayFunc
}
// Request returns the special task-queue HTTP request headers for the current
// task queue handler. Returns an error if called from outside a delay.Func.