Bug 17689: Add the Koha::Checkout->is_overdue method

Bug 17689: Add the Koha::Checkout->is_overdue method

This patch adds a new method Koha::Checkout->is_overdue and provide tests
to cover it.
The goal is to behave like GetItemIssues set the 'overdue' flag to
issues.
I don't understand why the existing GetItemIssues truncate dates to
minutes, so I did not recreate this behavior.

Test plan:
  prove t/db_dependent/Koha/Checkouts.t
should return green

Signed-off-by: Mika Smith <mikasmith@catalyst.net.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>