[FIX] purchase_order_type: preserve order type when matches company

Before this fix, when onchange event for company_id was fired, type for
purchase order was always set, then sometimes overwritten, even if
former order type was compatible with the new company.

This cause an incompatibility with e.g. purchase_order_type_dashboard.
When PO creation form is accesed coming from PO dashboard, selected
order type was actually igored, because onchange for company was fired
and order type overwritten. This fix it.

purchase_order_type/models/purchase_order.py

@@ -64,4 +64,8 @@ def _default_order_type(self):
 
     @api.onchange("company_id")
     def _onchange_company(self):
-        self.order_type = self._default_order_type()
+        if not self.order_type or (
+            self.order_type
+            and self.order_type.company_id not in [self.company_id, False]
+        ):
+            self.order_type = self._default_order_type()

purchase_order_type/tests/test_purchase_order_type.py

@@ -75,6 +75,9 @@ def test_purchase_order_change_company(self):
         order.onchange_partner_id()
         self.assertEqual(order.order_type, self.type2)
         order._onchange_company()
+        self.assertEqual(order.order_type, self.type2)
+        order.write({"order_type": False})
+        order._onchange_company()
         self.assertEqual(order.order_type, order._default_order_type())
 
     def test_purchase_order_type_company_error(self):