Bug 18361: Koha::Objects->find should accept composite primary keys

Bug 18361: Koha::Objects->find should accept composite primary keys

Changes the $id parameter to an array. (IssuingRule has three keys.)
The build_object method in TestBuilder.pm has been adjusted to pass
multiple primary key values to find.

Also adjusted the POD section to show more clearly that we accept
the same parameters as DBIx ResultSet does.

Test plan:
Run t/db_dependent/Koha/Object.t
Run t/db_dependent/Koha/Objects.t
Run t/db_dependent/TestBuilder.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>