Add proxy fix support for x-forwarded-prefix

[ilansh]

This header can be useful for apps where the root path is different behind proxy, for instance in my case using the traefik proxy - https://docs.traefik.io/basics/

[davidism]

If the proxy is stripping the prefix and adding this header, we're supposed to detect this header and add it to the URL before matching happens?

[davidism]

There are some other issues open about ProxyFix not supporting all the Forwarded headers (including the real Forwarded header). If you have time to create a PR that would be very helpful.

[ilansh]

No problem, I'll try to make some time for a new PR.

[ilansh]

As far as I understand, url matching is on the stripped urls (not adding the prefix from the header).
Url building adds the prefix to the returned url.