Fix rounding bug with pre-tax product % coupon

When applying a Product % Discount coupon before tax, there is a rounding bug.

Coupon details: http://cl.ly/image/2G35473M0v3P

Example checkout showing rounding bug: http://cl.ly/image/28082b2Q0y2a

This patch rounds the discount before adding it in the same way the discount is rounded for a `'percent'` discount (i.e. cart % discount).