Patch for product class discounts with varying price per item

Before:

If you had two products in a class with different prices, e.g., 3.75, and 2.75, and applied a discount to 'n' items of that class, this code would calculate the discount on the price of the last item in the cart.

So, if I added 3 @ 3.75, and 3 @ 2.75, and had a 10% discount of 6, I would get a discount of 10% off of (6 * 2.75), not the proper discount.

This patch fixes that issue.