messages.go
func (m *messagePageIterator) Next(ctx context.Context) (*MessagePage, error) {
	mp := new(MessagePage)
	err := m.p.Next(ctx, mp)
	if err != nil {
		return nil, err
	}
	m.p.SetNextPageURI(mp.NextPageURI)
	return mp, nil
}
func (m *messagePageIterator) Next(ctx context.Context) (*MessagePage, error) {
	mp := new(MessagePage)
	err := m.p.Next(ctx, mp)
	if err != nil {
		return nil, err
	}
	m.p.SetNextPageURI(mp.NextPageURI)
	return mp, nil
}
// Next returns the next page of resources. If there are no more resources,
// NoMoreResults is returned.