Bug 35796: Properly set patron_expiration value

Bug 35796: Properly set patron_expiration value

To test:
1) Set an expiration on the password in a patron account and save.
2) Go to patron details (moremember.pl) and confirm the password has an expiration.
3) Edit the patron account without touching anything and save the account.
4) Go back to patron details (moremember.pl) and note that the expiration on the password has been removed.
5) APPLY PATCH
6) Try 1 - 4 again except this time password expiration is not removed.

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>