Double urls produced by rewriting of singlesignon urls

Singlesignon implements a special form of goto that prepends the fully qualified master domain url, see e.g. this call in singlesignon.module:

      singlesignon_goto($master_url . url('singlesignon/initial_check', $query));

If the multidomain_force_default_domain variable is set to TRUE, and hence all urls are rewritten, we end up with the singlesignon urls getting double protocols and domains.

So we need to skip singlesignon paths when rewriting. Patch attached.