sendAction(recipientId, action, options) {
    const recipient = this._createRecipient(recipientId);
    return this.sendRequest({
      recipient,
      sender_action: action
    });
  }
Send an action.
To send a typing indicator in a more convenient way, see the .sendTypingIndicator method.
@param {Recipient|String} recipientId Recipient object or ID.
@param {String} action One of 'mark_seen', 'typing_on' or 'typing_off'
@param {SendMessageOptions} [options] NOT USED.