Code review comment for lp:~icsergio/openobject-italia/61_imp_withholding_tax

Hi Lorenzo,
I did a little improvement, and from some tests it seems working well (with more service lines and no-tax lines too).
The amount are calculated in this way:

amount paid / invoice debit = % paid of debit
* withholding amount = % paid of withholding amount
* withholding rate = withholding amount to pay to tax authorities