Bug 23207: Automatic checkin items should never be overdue

Bug 23207: Automatic checkin items should never be overdue

This patch makes Koha::Checkouts->automatic_checkin pass the date_due as
return date to AddReturn.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Checkouts.t
=> FAIL: The feature is not working properly
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>