Bug 18936: (follow-up) Fix checkouts endpoint and its tests

Bug 18936: (follow-up) Fix checkouts endpoint and its tests

- Fixes a bug caused by the refactoring in
Koha/REST/V1/Checkouts.pm (run the tests without this patch and you
will see the tests fails because 'rule_name' parameter was
forgotten).

- Changes the tests to use the new Koha::CirculationRule objects
  instead of directly using issuingrules DB table

Signed-off-by: Minna Kivinen <minna.kivinen@hamk.fi>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>