code: function mustBeString(filter, keyword, field) {
    if (typeof filter[field] !== 'string') {
      return Bluebird.reject(new BadRequestError(`Attribute "${field}" in "${keyword}" must be a string`));
    }
  
    return Bluebird.resolve();
  }
docs: Verifies that filter.field is a string
  @param filter
  @param keyword
  @param field
  @returns {Promise}