[web] Fix HEAD request handling (closes #3677949)
Don't special-case HEAD in CubicWebRequestBase.is_client_cache_valid so
we don't return garbage. HEAD is supposed to return the actual headers
that a GET would return, which we can't do if we bypass all the normal
cubicweb logic. In particular, we no longer always return 200, and
return the right Content-Length/Content-Type/...