Adding item to cart should not create a new item, but increase quantity

[UnrulyNatives]

In the code presented in Store Items For a Cart the logic simply adds a DB record in the table.
This works well when adding a first itemable product.

Should be: If a buyer press the same button again, the logic should increase quantity.
Now: A new entry in the CartItem model is created.

Also, an inctrease quoantity feature should be implemented.

[milwad-dev]

Yes, we add a new method in the Laravel Cart update.

[milwad-dev]

#16