Bug 22421: (follow-up): Check issue during add_credit

Bug 22421: (follow-up): Check issue during add_credit

Additional handling for the case where a credit is added with an
issue_id after the issue is returned.

add_credit will keep the same method signature, where the issue_id
parameter may now refer to a current issue or an old issue. It will then
internally determine whether it is a current or old checkout and handle
it appropriately.

If the caller is updating an account line, or adding a credit to refund
an existing debit, they may not need to know whether it is for a current
checkout or an old checkout. In this case, they can use the account
line's ->checkout accessor to pass in the appropriate issue_id
regardless of whether it is a current or old checkout.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>