Bug 25020: Preserve time part when batch extending due dates

Bug 25020: Preserve time part when batch extending due dates

When selecting a new hard due date, we should keep the time part of the
original checkouts.

Test plan:
1 - Checkout an item specifying a date due
2 - Checkout an item without specifying a date due
3 - Use Tools->Batch extend due date
4 - Select a hard due date
5 - On the confirmation screen you should see that the time part has
been kept
6 - Confirm
7 - Make sure the correct values hava been inserted in DB
8 - Try now using the other option, give a number of days
9 - Repeat 4-7

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>