l10n_it_withholding_tax: wrong WT entry when paying multiple invoices

[eLBati]

Affected versions:

• [ ] 14.0 ?
• [ ] 16.0 https://github.com/OCA/l10n-italy/pull/4082

Steps to reproduce the behavior:

1. Create 2 supplier invoices like the following:

2. Using account_reconcile_oca, create a payment line like the following:

3. Start reconciliation and set the amounts like this:

4. Validate

Observed behavior

The following entries are generated:

details:

and the 2 invoices are paid

Expected behavior

2 WT entries are generated, 1 per invoice. Total balance of "260200 Debiti per ritenute da versare" must be 40