code: function handleOptions(req, res) {
    var acrh = req.headers['access-control-request-headers'];
    var origin = req.headers['origin'];
    if (acrh) res.setHeader('Access-Control-Allow-Headers', acrh);
    if (origin) res.setHeader('Access-Control-Allow-Origin', origin);
    res.end();
  }
docs: Echo CORS headers.