Fix shipping & tax calculations in javascript when free shipping prom…

[mrbrdo]

…otion is applied

[nciemniak]

@mrbrdo can you give me details on how to reproduce the bug?

[mrbrdo]

@nciemniak
Create a promotion (coupon code...) for example on order above $100, applies free shipping. Use a default shipping method that has a price, and another shipping method which is free (like in-store pickup for example). Then try to check out with an order above $100 and watch carefully the cost of shipping listed in the checkout steps, and try to switch between both possible shipping options. I think that should do it. In my setup I use an EU VAT rate which is included in the price (might be relevant or not, don't remember).
Too bad I did not put more details in the commit message, I have to do that in the future.