[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.
usr/lib/python2*/*-packages/cubicweb/devtools/
usr/lib/python2*/*-packages/cubicweb/skeleton/
usr/lib/python2*/*-packages/cubicweb/test
usr/lib/python2*/*-packages/cubicweb/dataimport/test
usr/lib/python2*/*-packages/cubicweb/entities/test
usr/lib/python2*/*-packages/cubicweb/ext/test
usr/lib/python2*/*-packages/cubicweb/server/test
usr/lib/python2*/*-packages/cubicweb/sobjects/test
usr/lib/python2*/*-packages/cubicweb/hooks/test
usr/lib/python2*/*-packages/cubicweb/web/test
usr/lib/python2*/*-packages/cubicweb/etwist/test