[5.0] [account] Reconciliation: wrong additional entry included in the reconciliation

Hi!
5.0 latest bzr.
Reconciling a supplier invoice in the domestic currency with a write-off > 0 results in a wrong residual amount. That's because the reconciliation code wrongly identifies this case as an exchange rate income/expense kind of write-off.

Example case:
supplier refund 1839.97
payment to supplier 14157.05
payment to supplier 16597.89
supplier invoice 17590.26
supplier invoice 15004.63
sum debit: 32594.91 sum credit: 32594.89

Reconciliation wizard with a write-off of 0.02. The invoice with the total of 15004.63 will have a residual of -0.02 because both sides of booking the write-off will be included in the invoice's payments.

Patch included.