Bug 23770: (follow-up) Use ->to_api in /cities

Bug 23770: (follow-up) Use ->to_api in /cities

This patch aims to prove the behaviour does not differ. So you can run
the tests for the endpoints, and the behaviour remains.

The mappings and methods cannot be removed until $c->objects->search is
fixed to make use of them as well. That will happen once all used
Koha::Object-based classes implement the mappings.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/cities.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>