Added call to $store->save() method which triggers saveCart()

[alexrayan]

Item updating is not working in laravel-cart due to the fact that there is no call to saveCart() method following update item methods (either in Cart or in Item) which leads to the updates being lost next time Cart __construct() is being called.
Added call to $store->save() method which triggers saveCart() in persistent storage (mainly for laravel-cart) on cart item update (from within Cart model and from Item model). Added an empty public save() method to Runtime storage for reference.

[alexrayan]

There is a better way of fixing the item update not saving issue by adding:

$this->store->insertUpdate($this);

to the update() method in Item.

This, however, wouldn't fix the issue of item not updating in the following case:

Cart::item($id)->price = 1;