Resolves reactioncommerce/example-storefront#462
Impact: major
Type: bugfix

Issue

Adding additional quantities of an item that is already in your cart does not trigger a subtotal recalculation, leaving the numbers in the cart preview incorrect. For example, the following image, 1 item was added, then as a second action, another item was added to the cart. The subtotal should be $39.98 ($19.99*2), but it still displays $19.99.

Solution

Add a recalculation of the subtotal inside addItemsToCart util.

Testing

1. Start reaction-platform
2. Add any quantity of items to a cart
3. See the subtotal is correct in the mini-cart dropdown
4. Add any number of the same items to the cart
5. See that the subtotal has updated and the calculation is correct
6. Also see that subtotal is correctly calculated when using +, - and remove buttons inside the cart itself