Bug 22200: Add credit when forgiving overdue

Bug 22200: Add credit when forgiving overdue

Prior to this patch the exemptfine option for_FixOverduesOnReturn would
directly set the overdue amount to 0 rather than creating an appropriate
credit of the type forgiven and offseting it against the debt.

Test Plan:
1) Find a checkout that is overdue with fines
2) Check the item in with the forgive fines option checked
3) Note that the users account details now shows the overdue as forgiven
and a forgiven credit is added which matches the overdue amount (and is
applied against it, i.e. both overdue and forgiven lines have 0
amountoutstanding).

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>