Test to see if a better coupon exists already

If we don't find the valid promo in all our places it could be
successfully applied, but it *is* still on the order, it was accepted by
the order but a better one was used instead
hey @cbrunsdon I think this will break any time a order has a promotion with code `nil` we cant do `nil.downcase` to a `to_s` here is required I guess.

or you could probably do something like this `order.promotions.with_coupon_code order.coupon_code.downcase`, wdyt?