_authenticate(auth, request) {
    if (!auth) return true;
    const credentials = this._getCredentials(request.headers.authorization);
    if (credentials === `${auth.username}:${auth.password}`) return true;
    return false;
  }
@param {!{username:string, password:string}} auth
@param {!IncomingMessage} request
@return {!boolean}
@private