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.
I don't know. I am pretty new to this part of stripe. Do you know how to test against older versions of the stripe API?