Bug 29464: Make GET /acquisitions/orders honour sorting

Bug 29464: Make GET /acquisitions/orders honour sorting

When bug 20212 was pushed, we lost (didn't dig enough to find out how)
the sorting feature for this route.

This is mainly because biblio+biblioitems filtering prevents the route
to use plain $c->objects->search, so it has baked a custom version of
it. In this case, it missed the call to the dbic_merge_sorting helper,
that takes care of reading the query parameters and inject the sorting
attributes to the resultset.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/acquisitions_orders.t
=> FAIL: Tests fail, sorting doesn't actually work
3. Apply this patch
4. Repeat 2
=> SUCCESS: Sorting works!
5. Test sorting the pending orders on parcel.tt
6. Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>