Bug 37540: Ensure datetime is a string before enqueueing pseudonymization

Bug 37540: Ensure datetime is a string before enqueueing pseudonymization

To test:
1 - Enable Pseudonymization in system preferences
    NOTE: See bug 28911 for bcrypt setup
2 - Issue an item to a patron
3 - View the patrons checkouts
4 - Check the box under 'Renew'
5 - Renew selected items
6 - Internal server error
7 - Apply patch
8 - Restart all
9 - Repeat 4&5
10 - Success!

Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>