Bug 24321: Make dbic_merge_sorting accept a result set as parameter

Bug 24321: Make dbic_merge_sorting accept a result set as parameter

This patch makes dbic_merge_sorting accept a result set as parameter and
solves a FIXME in _build_order_atom.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>