…criptions.

Users can contact Apple customer support to cancel an autorenewable
subscription.  After doing so, validating a receipt for the subscription
will return a response with a 'cancellation_date' field.

From the Apple docs:

  "For a transaction that was canceled by Apple customer support, the time and
  date of the cancellation.  Treat a canceled receipt the same as if no purchase
  had ever been made."

Closes nov#18

Value used to be return as milliseconds, now it's returned as a formatted date
string (with extra fields for milliseconds and PST-formatted date).

Closes nov#17