XHR filters multiple HTTP response headers of the same type

If an XHR response contains multiple HTTP headers of the same type, only one of them is available in `XMLHttpRequest` and `fetch` APIs. For example, `response.headers.getAll('Set-Cookie')` will return an array of length 1, even if there are multiple `Set-Cookie` headers in the response.

The reason is that headers are currently passed to the JS side as an object, hence multiple headers will overwrite one another.