Bug 34117: Remove side effect from get_expiry_date

Bug 34117: Remove side effect from get_expiry_date

If get_expiry_date is passed a DateTime object as a parameter,
it modifies and returns the original object. When memberentry.pl
prefills the input fields for duplicating a patron, it passes the
enrollment date object to get_expiry_date. This causes the enrollment
date object to be modified with the expiry date value.

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

To test:
1. Have or make a patron
2. Duplicate that patron
3. Before saving the new patron, scroll down to Registration Date and
   see that it's defaulting to a date in the future.
4. Apply patch and restart_all
5. Try duplicating a patron again
6. Registration Date should correctly set to today

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>