Fix instock update in pre-update hook of order detail model

[stefanheppenheimer]

The order detail model automatically updates the instock value of the related article detail in a pre-update hook. Unfortunately these updates will not be persisted along with the order detail. This is due to the fact that according to the doctrine documentation changes made to an entity inside a pre-update hook will not be committed in the same flush event as the "host" entity. Therefore these changes will stay "pending" until the next (global) flush event. This makes the automatic update of the instock value unpredictable and leads in fact to a lot of problems especially in the order cancellation. Therefore we should fix the automatic update and send Shopware a pull request.

http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/events.html#preupdate

Shopware 5.2.12 (Order Detail Model)
https://github.com/shopware/shopware/blob/v5.2.12/engine/Shopware/Models/Order/Detail.php#L607-L640