Bug 33974: (follow-up) _order_by is not always an arrayref

Bug 33974: (follow-up) _order_by is not always an arrayref

The loop through the `_order_by` query parameter occurences introduced
by this patchset was naive regarding the possible scenarios.

When there's only one parameter passed, it shouldn't be expecting an
arrayref, but a scalar. This patch deals with that in the simplest way.

To test:
1. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Koha/REST/Plugin/Objects.t
=> FAIL: Tests are failing
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>