Bug 26896: Enable SIP option holds_block_checkin to actually block checkin of items...

Bug 26896: Enable SIP option holds_block_checkin to actually block checkin of items with holds

It appears that at some point, the actual blocking of checkins on items
with holds got lost from the patch set. This slipped by because the sip
server still outputs everything correctly, but the unit tests don't
actually test to see if the checkout still exists afterward!

Test plan:
1) Apply this patch
2) Prove t/db_dependent/SIP/Message.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>