Bug 26108: Only call the plugin hook if a return took place

Bug 26108: Only call the plugin hook if a return took place

This patch makes the hooks be called only if the checkin is effective
(i.e. if it is not rejected for some reason.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Plugins/Circulation_hooks.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>