This breaks --proxy-path-passthrough
However, we still need this when that code is disabled as many apps will
issue absolute redirects and expect the proxy to rewrite as appropriate.
e.g. Go's http.Redirect will rewrite relative redirects as absolute!
See https://golang.org/pkg/net/http/#Redirect
This is mostly so we don't have to do any wacky patching but it also
makes it so we don't have to keep checking if the request is a web
socket request every time we add middleware.
