Bug 19686: (follow-up) Add to_api param for completeness

Bug 19686: (follow-up) Add to_api param for completeness

This patch adds (yet) another param to objects.search: a reference
to a to_api function to be applied when processing the search results
for output.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/REST/Plugin/Objects.t
=> SUCCESS: Test count raised, and tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>