Migrate from X-Forwarded-For to Forwarded

[GoogleCodeExporter]

X-Forwarded-For has been standardized to Forwarded

This has the advantage that we get the original protocol and therefore can tell whether the original protocol was HTTPS or HTTP

Forwarded: by=<identifier>;for=<identifier>;host=<host>;proto=<http|https>

We must pay attention that we do this only if we are behind a forward proxy that sets the header and not when we are directly connected to the internet.

See HTTP/1.1 just got a major update. and RFC 7239: Forwarded HTTP extension.