Update index.js

[M-zhar]

Setting Vary: Origin Header for All Requests:

The res.setHeader('Vary', 'Origin') line ensures that every response includes the Vary: Origin header, regardless of whether it's a CORS or non-CORS request.

//CORS Middleware for Requests with Origin:
The dynamic cors middleware is applied only if the originCallback function validates the origin.

//Non-CORS Requests:
For requests without the Origin header, the middleware still sets the Vary: Origin header and proceeds without invoking the cors middleware.

[M-zhar]

Differentiating in cors-request and non-cors request