Bug 24733: created_by param should be an arrayref

Bug 24733: created_by param should be an arrayref

We allow for the user to add multiple users to this field, and the
search function expects and array. We have been treating it as a scalar

TO test:
1 - Have an open basket in acquisitions
2 - Add to basket - from an existing order (copy)
3 - Enter a name in the 'Basket created by' field and select a user
4 - Hit search
5 - ISE (Can't use string ("1") as an ARRAY ref while "strict refs" in use at /usr/share/koha/lib/C4/Acquisition.pm line 2482.)
6 - Apply patch
7 - Repeat 1-4
8 - Success!
9 - Test with mutiple 'created by' entries as well

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>