sendRequest(body, endpoint, method) {
    endpoint = endpoint || 'messages';
    method = method || 'POST';
    return fetch(`https://graph.facebook.com/${this.graphApiVersion}/me/${endpoint}?access_token=${this.accessToken}`, {
      method,
      headers: {
        'Content-Type': 'application/json'
      },
      body: JSON.stringify(body)
    })
    .then(res => res.json())
    .then(res => {
      if (res.error) {
        console.log('Messenger Error received. For more information about error codes, see: https://goo.gl/d76uvB');
        console.log(res.error);
      }
      return res;
    })
    .catch(err => console.log(`Error sending message: ${err}`));
  }
@param {Object} body The request body object.
@param {String} endpoint Messenger API endpoint
@param {String} method HTTP method.
@returns {Promise}