Bug 25421: Make ->pickup_locations only return an arrayref

Bug 25421: Make ->pickup_locations only return an arrayref

This patch makes Koha::Item->pickup_locations and
Koha::Biblio->pickup_locations always return an arrayref.

Test are adjusted to reflect this:

1. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Biblio.t t/db_dependent/Koha/Item.t
=> SUCCESS: Tests pass!
2. Apply this patch and repeat 1
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Didier Gautheron <didier.gautheron@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>