Bug 13841 - Returns via backdating or SIP2 can create new fines with an amount outsta...

Bug 13841 - Returns via backdating or SIP2 can create new fines with an amount outstanding of 0

Due to the way AddReturn and UpdateFine work, if you pass in a return
date to AddReturn, and that item has a calculated fine of 0, Koha will
insert the 0 amount fine into accountlines.

This is good behavior when we want to update an existing fine, but bad
behavior if there was no fine to begin with!

Test Plan:
1) Apply the unit test patch
2) prove t/db_dependent/Circulation.t
3) Note the test fails
4) Apply this patch
5) prove t/db_dependent/Circulation.t
6) Note the test passes

Signed-off-by: Jen DeMuth <jdemuth@roseville.ca.us>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Confirmed the problem in the staff interface and that the patch
fixes it. Also tested that fines still get calculated.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>