Bug 34435: Remove side effect from get_password_expiry_date

Bug 34435: Remove side effect from get_password_expiry_date

If get_password_expiry_date is passed a DateTime object
as a parameter, it modifies and returns the original
object. This can create possible side effects.

This patch modifies get_password_expiry_date to clone the
DateTime object that it receives as a parameter and
return the modified clone, so that object references can
be passed in safely.

To test:
prove t/db_dependent/Koha/Patron/Category.t

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>