Bug 24366: Make sure accessors are used on the right object

Bug 24366: Make sure accessors are used on the right object

This patch makes the Koha::Biblio->to_api method try the accessors on
$self first, and fallback to $self->biblioitem on those he can't.

This way it won't happen that (for example) ->holds will be tried
against ->biblioitem.

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>