code: function sendSenderAction (recipient, sender_action) {
    return new Promise (async (resolve, reject) => {
      if (!sender_action) {
        reject('sender_action required');      
      }
  
      let payload = {
        'sender_action': sender_action
      };
  
      try {
        let response = await this.callSendApi(recipient, payload);  
        resolve(response);
      } catch (e) {
        reject(e);
      }
    });
  }
docs: Sends a sender action via the [Send API](https://developers.facebook.com/docs/messenger-platform/reference/send-api).
  @param  {Object}  recipient  An object that describes the message recipient in the format: `{<id_type>: <id>}`.
  For example, sends to a PSID would be `{'id': 123456}`, to a phone number `{'phone_number': '+1 (408) 444-4444'}.
  @param  {String}  sender_action  The sender action to send. Must be `typing_on`, `typing_off`, or `mark_seen`.
  @return {Promise<Object>}  The API response
  @memberof  Client#
  @example
  let recipient = {'id': '57024957309673'},
  sender_action = 'mark_seen';
  Client.sendSenderAction(recipient, sender_action)
  .then(res => {
  console.log(res); // {"recipient_id": "1008372609250235"}
  });