Add BehindProxy filter

[thekid]

Rewrite request URI if behind a reverse proxy

On Apache:

<VirtualHost service.example.com:443>
  ProxyPass / http://service.example-backend.lan:8080/
</VirtualHost>

In your service:

$filter= new BehindProxy(['https://service.example.com/' => '/']);

See also #40 - especially the research and discussion as to why relying on a complicated mix of X-Forwarded-* headers and a trusted proxy list seems to be total overkill; hence this simpler solution.

[thekid]

Released in https://github.com/xp-forge/web/releases/tag/v3.0.0