validateAllowedHeaders(headers) {
        if (!headers) {
            return undefined; // to indicate AllowedHeaders do not exist
        }
        if (!Array.isArray(headers) || headers.length === 0) {
            return errors.MalformedXML;
        }
        const result =
            this.validateAllowedOriginsOrHeaders(headers, 'AllowedHeader');
        if (result instanceof Error) {
            return result;
        }
        return true;
    }
_validator.validateAllowedHeaders - check values of AllowedHeader's
@param {string[]} headers - array of AllowedHeader's
@return {(Error|true|undefined)} - Arsenal error on failure, true if
  valid, undefined if optional AllowedHeader's do not exist