Bug 24168: (bug 23116 follow-up) AllowHoldPolicyOverride allows a librarian to almost...

Bug 24168: (bug 23116 follow-up) AllowHoldPolicyOverride allows a librarian to almost place a hold on an item already on hold

This patch actually fixes the issue described on bug 23116.

Test plan:
See bug 23116 and comment 5. Important to note that the later comparison with
itemAlreadyOnHold assumes that the variable is a string.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>