Bug 18046: CAS logout infinite redirect

Bug 18046: CAS logout infinite redirect

To test (You need a CAS server and CAS configured in Koha)

1/ Login using CAS in Koha
2/ Logout in Koha
3/ Notice you get redirected again and again
4/ Apply patch
5/ Login with CAS, then logout
6/ Notice logout works, but no longer infinitely redirected

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Patch has been in production use for several months
on several instances. Fixes a critical bug.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Looks like a typical workaround, but evidently works.
Not tested with CAS.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>