Wrong shipping tax calculation if order level discount

Description

Preconditions

• Sales tax and shipping tax 20%
• Order level discount 10%

Step to reproduce

1. Add a line item to a cart whose price is $30.50 and quantity = 1.
2. Fulfill the order level discount.
3. Select shipping method. It has a shipping cost of $20.
4. Go to the checkout page.

Expected:
The tax total is $9.49 = $5.49 sales tax + $4 shipping tax

Actual:
The tax total is $9.09 = $5.49 sales tax + $3.6 shipping tax

Reference to thread http://world.episerver.com/forum/developer-forum/Episerver-Commerce/Thread-Container/2017/10/problems-with-vat-calculation-when-using-discount-code