Set date on PayInvoice.ask transition

Otherwise when using a different currency it raises an error because an
exchange rate can not be found for an empty date.
issue6053
review32821002

invoice.py

@@ -2546,6 +2546,7 @@ class PayInvoice(Wizard):
                 if not x.reconciliation]
 
         default['amount'] = self.start.amount
+        default['date'] = self.start.date
         default['currency'] = self.start.currency.id
         default['currency_digits'] = self.start.currency_digits
         default['company'] = invoice.company.id