Don't redirect to from HTTP to HTTPS

Let applications redirect basing on `X-Forwarded-Proto` header.

Example of a usecase where this would be better than redirecting by
default: dokku running at `apps.<root-domain>`, with a wildcard
certificate for `*.<root-domain>` set up. Applications which require SSL
could be deployed as `<subdomain>.<root-domain>` (by setting DNS up to
point to dokku host and using custom domain). Regular applications could
be deployed as usual.