Added a call to save to a storage on item update()

[alexrayan]

Neither Cart::update($id, 'price', $price); nor Cart::item($id)->update('price',$price); triggers saving to a storage, therefore on next Cart construction the updates are lost.

I initially wanted to add another method to a storage save() which would call saveCart() protected method since with the above approach the following won't trigger update:

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

But adding a $this->store->insertUpdate($this) within an Item object is better, since attribute value reassignment doesn't really tell us that the item has been updated and saved.

Regards,
Alex