When 'stock_dropshipping' was installed, in a sale order,
choosing a product being stockable + buy + MTOs returned
the "Not Enough Stock" warning, while it shouldn't.
It's because the according method `_check_rounting` was
overriden in this module, with a @api.one, which aggregates, while
the method expects to return a simple boolean even for a list of
multiple ids.
Converting the method to @api.multi solves the issue.
opw-639426
When selectiong a dropshipped product in a sale.order, the user should not be
warned that the product is not available in stock (as it never goes to the stock
anyway but directly from the supplier to the customer).
Factorise verification in _check_routing method.
Denis Ledoux
Ravi Gohil