[FIX] account: set 'multi' field to False for payments generated when choosing to register payment for multiple invoices with distinct partners

This field is used by the multiple payment wizard; the payments it generates are always individual and should never have multi=True. Before this commit, they did, because of the values given in context when calling the payment wizard.

addons/account/models/account_payment.py

@@ -197,6 +197,7 @@ class account_register_payments(models.TransientModel):
             'currency_id': self.currency_id.id,
             'partner_id': invoices[0].commercial_partner_id.id,
             'partner_type': MAP_INVOICE_TYPE_PARTNER_TYPE[invoices[0].type],
+            'multi': False,
         }
 
     @api.multi