code: function updateUserSettings(userId, settings, overwrite, callback) {
          return _updateUserObjectField(userId, ['settings'], settings, overwrite)
              .then(function (userData) {
                  return userData.settings;
              })
              .nodeify(callback);
      }
docs: Updates the provided fields in the settings.
  @param {string} userId
  @param {object} settings
  @param {boolean} [overwrite] - if true the settings for the key will be overwritten.
  @param {function} [callback]
  @returns {*}