Wrong shipping tax when applying order promotion to zero for cart with different items

Description

Precondition: Configure shipping tax for item with rate = 50%
Steps to reproduce:

1. Create an order promotion "Discount of order value" or "Buy product and get discount on order" with amount off = 3000.
2. Add to the cart 3 items that satisfy the promotion and have a shipping tax.
3. Go to the checkout page.
4. Select a shipping address that has tax.

Expected: Commerce calculates and shows shipping tax correctly.

• Following the way to calculate shipping tax, refer to link on issue COM-1090, the shipping tax for order total should be zero: http://www.skatteverket.se/rattsinformation/skrivelser/arkiv/ar/2002/2002/skrivelser20020913.4.18e1b10334ebe8bc80001578.html#

Actual: Shipping tax = shipping tax on order * quantity. So it exceeds the shipping cost. In this case shipping tax = 12*50%*3= 18.