Proxy Protocol Port not used to determine Access Scheme

If deis router is running with PROXY protocol enabled the value of `$proxy_protocol_port` is not regarded when determining the `$access_scheme` which ends up in the `X-Forwarded-Proto` header sent to backends.

This matters if SSL is terminated on a load balancer in front of the deis router, which would then send on requests to the router's http port, but indicate in the PROXY protocol header, that the destination port was 443.

The deis router could find this value from the nginx var `$proxy_protocol_port` that is supported since nginx 1.11.0.