Prefer non-standard headers like X-Forwarded-Proto, X-Forwarded-Host and
X-Forwarded-Port over the standard Forwarded header to maintain
backwards compatibility.

If a port is not specified neither in Host nor in forwarded headers but
it is specified just with X-Forwarded-Port, use its value in base urls
for redirects.

Forwarded header is defined in rfc7239.

X-Forwarded-Port is a non-standard header. Here's a description copied
from "HTTP Headers and Elastic Load Balancing" of AWS ELB docs:

> The X-Forwarded-Port request header helps you identify the port that
> an HTTP or HTTPS load balancer uses to connect to the client.

Signed-off-by: Michal Minář <miminar@redhat.com>