Proxied requests need to control what information they send upstream about galactory

[briantist]

This was a little hard to reproduce at first, because if the cache is in use and populated then the problem doesn't happen because the request doesn't hit the upstream.

Also, this seems to be due to the Accept header only and I think that we should leave user agent alone, that way upstreams can accurately gather their own stats.

So this should be solvable just by removing / resetting the Accept header.

[briantist]

There was one other issue: the Accept-Encoding header from the browser supported br (brotli), and with no brotli package installed, the compressed response could not be decompressed.