Bug 12959 - Check in forgets to remember arbitrary backdating of returns setting

Whenever a librarian is prompted with a form during the checking process
( hold to fill, transfer, etc ) the date to backdate returns to is forgotten!

Test Plan:
1) Begin returning items with a remembered backdate
2) Check in an item that need transfered or has a hold
3) Complete the action requested
4) Note your set return date is now lost
5) Apply this patch
6) Repeat steps 1-3
7) Note your backdate is now remembered!

Signed-off-by: Sean McGarvey <smcgarvey@pascocountyfl.net>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>