WSGI proxy and Forwarded header

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.

Start by locating the WSGI proxy entry point and any existing proxy-header handling, then read RFC 7239 for the Forwarded header fields. Done means the proxy adds the header by default, supports the proposed opt-out behavior, and has coverage for the resulting request metadata.

Written by the indexing model from the issue text.