code: func (b *QuickReplyButton) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Type     string           `json:"type"`
  		ImageURL string           `json:"imageUrl,omitempty"`
  		Action   QuickReplyAction `json:"action"`
  	}{
  		Type:     "action",
  		ImageURL: b.ImageURL,
  		Action:   b.Action,
  	})
  }
docs: // MarshalJSON method of QuickReplyButton