Add support for `cancellation_date` field

[gmcnaughton]

It would be great if the receipt object would expose the cancellation_date field for auto-renewable subscriptions!

From the receipt fields 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.

[Paludis]

Thanks for adding it, seems like the original author of this gem has gone AWOL..