Make subscription cancelation behavior match Stripe

If you delete a subscription in stripe without setting at_end_period to true, it immediately deletes the subscription. If you fetch the customer and look at it's subscriptions, it is gone. However, with stripe-ruby-mock, the subscription stays around in the cancelled state. This fix corrects that issue.
Thanks for contributing. Since you're changing an existing test, I need to double check - did Stripe change its behavior, or was this specified behavior never correct in the first place?