Upstream removed the ability to consistently use a custom InventoryHolder,
However, the implementation does not use an InventoryHolder in any form
outside of custom inventories.
We can take that knowledge and apply some expected behavior, if we're given
an inventory holder, we should use it and return a custom inventory with the
holder, otherwise, create an inventory backed by the intended inventory, as
per upstream behavior.
This provides a "best of both worlds" scenario: plugins with InventoryHolder's
will always work as intended in the past, those without will create implementation
based inventories.
Shane Freeder