ModifyResponseHeader rewrites response headers indiscriminately

[dtomcej]

The commit 624f918

Avoids an extra redirect by modifying the scheme on the location response header.

However, it does not check that the location referred to in the header is SSLHost.

In effect, it rewrites all location responses to https, even if the proxy implementing unrolled/secure is not handling the domain.

It prevents redirecting to third party http URLs.

The modification should also not rewrite the scheme if there is a port defined in the response header.

Related issue: traefik/traefik#5807

PR Incoming