Fix X-Forwarded-Foo behavior

Ping @blaix - Let me know if this works for you.

This fixes:
* a problem with how djproxy was calculating the xff header (it should be adding the client IP address, not the server's IP address!)
* a problem with where djproxy was attaching the XFF header (it should send it to the downstream server, not back to the client!)

and it also makes djproxy start sending an X-Forwarded-Host header to the downstream proxy.