Bug 33613: Use prop() to evaluate checkbox

Bug 33613: Use prop() to evaluate checkbox

To test:
1. Have an item with a replacement cost checked out to a patron
2. Set the ClaimReturnedChargeFee sys pref to "Ask if a lost fee should be charged"
3. Make a claim and check the box for charging.
4. Claim happens but no charge occurs.
5. Apply patch and try again.
6. A charge should now occur.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Hinemoea Viault <hinemoea.viault@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>