This fixes most of the issues people encounter when there are
multiple proxies in the request. The tricky thing is that there
is another header "Forwarded" that we don't recognize, but which
backends probably do, at least some of the time (since it is
from an actual RFC). The problem is that "Forwarded" does not
contain the ports, so Spring UriComponentsBuilder cannot use it
to rewrite links to a specific port. Since we do not support it
already, this change doesn't make things any worse, but the
corner case is there still.