[web] fix headers on 304 responses (closes #11466875)
The response MUST include the following header fields:
[...]
- ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request
https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
Unfortunately we weren't copying headers from what would have been the
200 response, but from the request.