Bug 21607: Make Koha::Account::Line->apply store credits as negative amounts

Bug 21607: Make Koha::Account::Line->apply store credits as negative amounts

This is a trivial patch, that makes offsets have 'amount' stored as
negative values for applied credits.

The behaviour is changed on the tests and adjusted in the code. To test:
- Run
  $ kshell
 k$ prove t/db_dependent/Koha/Account/Lines.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>