Bug 28424: Fix patron credits route (POST)

Bug 28424: Fix patron credits route (POST)

This patch makes the route follow the coding guidelines and thus:
- Return the generated object
- Use the status code 201 for the response

To test:
1. Apply the tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> FAIL: Tests fail. The reponse body and status is incorrect
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: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>