Bug 24459: Make ->to_api pass $params through

Bug 24459: Make ->to_api pass $params through

This patch makes the to_api method in Koha::Acquisition::Inovice and
Koha::Patron pass the passed parameters to the base class to_api method.
This is required for things like embedding related objects.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Patron.t \
           t/db_dependent/Koha/Acquisition/Invoice.t
=> FAIL: Tests fail!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>