[FIX] account,sale: Select correclty the delivery partner

To deduct the fiscal position, we should use the shipping partner.

The field `partner_shipping_id` is defined in the sale module.

Thus, we should choose this partner if the module sale is installed and if this field is set, or fallback to the original partner instead

addons/account/models/account_invoice.py

@@ -484,7 +484,7 @@ class AccountInvoice(models.Model):
                 account_id = pay_account.id
                 payment_term_id = p.property_supplier_payment_term_id.id
 
-            delivery_partner_id = self.partner_shipping_id.id or self.partner_id.address_get(['delivery'])['delivery']
+            delivery_partner_id = self.get_delivery_partner_id()
             fiscal_position = self.env['account.fiscal.position'].get_fiscal_position(self.partner_id.id, delivery_id=delivery_partner_id)
 
             # If partner has no warning, check its company
@@ -513,6 +513,10 @@ class AccountInvoice(models.Model):
             return {'domain': {'partner_bank_id': [('id', 'in', bank_ids.ids)]}}
         return {}
 
+    @api.multi
+    def get_delivery_partner_id(self):
+        self.ensure_one()
+        return self.partner_id.address_get(['delivery'])['delivery']
 
     @api.onchange('journal_id')
     def _onchange_journal_id(self):

addons/sale/models/account_invoice.py

@@ -84,6 +84,10 @@ class AccountInvoice(models.Model):
 
         return report_pages
 
+    @api.multi
+    def get_delivery_partner_id(self):
+        self.ensure_one()
+        return self.partner_shipping_id.id or super(AccountInvoice, self).get_delivery_partner_id()
 
 class AccountInvoiceLine(models.Model):
     _inherit = 'account.invoice.line'