Bug 24191: Make objects.search pass to_model to dbic_merge_sorting

Bug 24191: Make objects.search pass to_model to dbic_merge_sorting

This patch makes objects.search pass the to_mode method reference so it
can be used to map API attributes to column names when building the
sorting portion of the query.

To test:
1. Apply the regression tests on this patchset
2. Run:
  $ kshell
 k$ prove t/db_dependent/Koha/REST/Plugin/Objects.t
=> FAIL: Tests fail!
3. Apply the rest of the patchset
4. Run:
 k$ prove t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass! changes to the methods work as expected!
5. Run:
 k$ prove t/db_dependent/Koha/REST/Plugin/Objects.t
=> SUCCESS: Tests pass!
6. Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>