_attachCookies (request_) {
    const url = new URL(request_.url);
    const access = new CookieAccessInfo(
      url.hostname,
      url.pathname,
      url.protocol === 'https:'
    );
    const cookies = this.jar.getCookies(access).toValueString();
    request_.cookies = cookies;
  }
Attach cookies when available to the given `req`.

@param {Request} req
@api private