Right now get_item() is loading an item from the database directly. It doesn't
take advantage of our cache and it doesn't check if the item is already loaded in memory.
There is also another bug (or feature?) that it will let you load any item, even if the item is not related to the current order. I believe this is a bug, if somebody really wants to load any item regardless of the order they should use `WC_Order_Factory::get_order_item`.
Another bug is that items loaded with get_item() are not related to the order object, therefore any calls to Order::save() won't persist the changes made to the item.
This commits makes sure the item returned by get_item is loaded similarly like get_items() does, taking advantage of the cache and the $items protected property (chances are the item is already in memory, ready to be used).
If a given item is not found false will be returned. If item exists but it is not related to the current object it will return false as well (If this behaviour is wanted, I can easily change it load the item anyways instead of returning false).
By using $this->get_meta_data() which passed meta through the
WC_Data::filter_null_meta() method, rather than $this->meta_data
which still has null values set.
Payment tokens was introduced in 2.6 and was updated to use the new CRUD code while some of the CRUD system was still in flux.
While most things were correct, the prop handling for custom fields (like a card's last 4 digits) were directly calling meta functions, instead of delegating to the data store/parent.
This PR moves these props to `extra_data` and follows the same pattern as product types or order items. It also updates some version tags to 3.0.0. Finally, it adds an additional test for saving meta after a create which looks like it was lacking.
To Test:
* Run `phpunit`.
* Go to the "My Account" tab and add a new payment method. You need a payment gateway that supports this, like Simplify.
* Test the add a payment method flow.
* Make a test purchase using the saved payment method.