code: func (m *TextMessage) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Type       MessageType      `json:"type"`
  		Text       string           `json:"text"`
  		QuickReply *QuickReplyItems `json:"quickReply,omitempty"`
  	}{
  		Type:       MessageTypeText,
  		Text:       m.Text,
  		QuickReply: m.quickReplyitems,
  	})
  }
docs: // MarshalJSON method of TextMessage