[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.
Experiment with the search bar. Try queries like "card sometextualcontent" or
"card wikiid index" or "345".