[7.0] l10n_it_withholding_tax + account_vat_on_payment: wrong VAT entries

steps:

 - install account_vat_on_payment + l10n_it_withholding_tax
 - create a supplier invoice with 'Vat on payment ' and 'withholding tax'
 - add one line with 30 + 6.6 VAT (22%)
 - add withholdin amount = 6
 - pay it with a voucher of 30.60

Withholding tax entries are correct

VAT entry is wrong.
It is 5.52 (30.6 − (30.6 ÷ 1.22))
but it should be 6.6 (36.6 − (36.6 ÷ 1.22))

The desired entries are

                        dr--cr
bank------------------------30.6
IVA credito-------------6.60
acquisti----------------30
ritenute da versare---------6

acquisti--------------------30
IVA sospesa-----------------6.60
debiti------------------36.60