Bug 15906: [QA Follow-up] Correct a partial payment

Bug 15906: [QA Follow-up] Correct a partial payment

The amount in accountlines should be saved as a negative amount, but
amount outstanding is correct.

Note: This routine is actually used only once (in paycollect.pl). And is
only cosmetically tested in Accounts.t.

Test plan:
[1] Create a manual invoice, Fine, say 20.
[2] Click on the Pay button next to this fine, and pay 1.
[3] Pay again, using 'Pay amount' button, and pay 2.
[4] Pay again, using 'Pay' on the same line, the full remaining amount.
[5] Verify that you see -1, -2 and -17 on the Accounts tab.
[6] Run t/db_dependent/Accounts.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>