[FIX] account: Incorrect due date for credit notes

Steps to reproduce the bug:

- Create an invoice I
- Set the invoice date in the past and  add payment terms
- Validate I
- Create refund from I

Bug:

The invoice date is set as today but the due date was set in the past.

opw:2033438

closes odoo/odoo#34748

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>