Bug 24366: Merge biblioitem in Koha::Biblio->to_api

Bug 24366: Merge biblioitem in Koha::Biblio->to_api

This patch moves the bilbioitem object merge into the
Koha::Biblio->to_api method, as opposed to doing it in the controller.
This is an inevitable change as other endpoints might require to embed
biblio objects, and they need to carry the biblioitem information.

Also, if we merge the tables, the transition will be straightforward
(remove the tests this patch introduces, and merge the mappings from
Koha::Biblioitem into Koha::Biblio).

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Biblio.t \
           t/db_dependent/api/v1/biblios.t
=> SUCCESS: Tests pass! i.e. the new to_api method is tested, and the
API tests don't expect any behavior change.
3. Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>