Use the hostname from the request instead of the VirtualHost when redirecting

When configuring mod_auth_cas on a VirtualHost with several aliased domains,
mod_auth_cas would always generate redirections or service URIs pointing at
the VirtualHost ServerName, instead of the incoming request Hostname.

This breaks mod_auth_cas in a subtle way when using it in the context of a
VirtualHost setup such as the one described in
http://httpd.apache.org/docs/2.2/vhosts/mass.html#xtra-conf