messages.go
func (m *MessageService) SendMessage(from string, to string, body string, mediaURLs []*url.URL) (*Message, error) {
	v := url.Values{
		"Body": []string{body},
		"From": []string{from},
		"To":   []string{to},
	}
	if mediaURLs != nil {
		for _, mediaURL := range mediaURLs {
			v.Add("MediaUrl", mediaURL.String())
		}
	}
	return m.Create(context.Background(), v)
}
func (m *MessageService) SendMessage(from string, to string, body string, mediaURLs []*url.URL) (*Message, error) {
	v := url.Values{
		"Body": []string{body},
		"From": []string{from},
		"To":   []string{to},
	}
	if mediaURLs != nil {
		for _, mediaURL := range mediaURLs {
			v.Add("MediaUrl", mediaURL.String())
		}
	}
	return m.Create(context.Background(), v)
}
// SendMessage is a convenience wrapper around Create.