Backends should be compatible with redirects

[gbaz]

We'd like to add redirects to hackage-server to allow both efficient CDN usage and correct download statistics (haskell/hackage-server#698). However, downstream clients will need to be able to follow redirects. I checked and the HTTP backend isn't, at least (which breaks hackage-mirror-tool). That should be fixed, and we should investigate the other backends as well...