using already applied discount code (from session) to determine the _discount member of DiscountForm. Required so that when set_discount() is called from recalculate_cart(), the discount amount is recalculated based on the current cart. Fixes #132
Upmerge from stephenmcd/master
Allowing Discount Code deduction amount to be less than or equal to an item's cost, so it's possible to reduce the item cost to 0. (replacing < with <=)