WSGI proxy and Forwarded header

[faassen]

It would be useful if the WSGI proxy could say that it was there by adding a Forwarded header:

http://tools.ietf.org/html/rfc7239

I pick this one over older conventions like X_FORWARDED_HOST and X_FORWARDED_PROTO because this one is part of HTTP 1.1 now and has an official RFC.

I think the default should be for the WSGI proxy to add such a header, though it may be possible to turn it off.

A Forwarded header is useful for instance when the proxied HTTP server wants to generate complete URLs to itself. It can introspect the Forwarded header for the proto and host in order to get the right information.