Correct redirect if using incorrect path in URL: only rewrite path part

Correct redirect, as decribed on https://forums.sabnzbd.org/viewtopic.php?f=2&t=10443&p=95175#p95175
Only the path is rewritten, not the host (as it happened before)

So: http://192.168.1.111:8080/isnotthere now redirects to the correct URL http://192.168.1.111:8080/sabnzbd/