Filter out all receipts with "cancellation_date" != nil (#163)

[bizz84]

See #163. According to Apple docs:

Cancellation Date
For a transaction that was canceled by Apple customer support, the time and date of the cancellation.
ASN.1 Field Type 1712
ASN.1 Field Value IA5STRING, interpreted as an RFC 3339 date
JSON Field Name cancellation_date
JSON Field Value string, interpreted as an RFC 3339 date
Treat a canceled receipt the same as if no purchase had ever been made.

https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ReceiptFields.html

[vitovalov]

As mentioned in docs,

The Cancellation Date (cancellation_date) field is designed for use with auto-renewable subscription and non-consumable products.

It also applies to non-consumables. Then I'd like to know how to proceed if the user has bought an item ("no ads") and later cancelled it.

Initially I thought I need to make verify receipt call on every app launch to update the state of a product to "purchased" or "not purchased". Now reading this PR, I don't understand how this works.

If user returns a bought product, it's like he didn't pay for it. Therefore the app should return to the state when the product was not bought. How can we accomplish this?

[harryworld]

@vitovalov This is how I handle it now.

1. Force Refresh the receipt at launch
2. Updated receipt will have cancellation_date (Available in ASN.1 Field Type 1712)
3. This PR will verify cancellation_date and return as .notPurchased

Now, my question to @bizz84
If I refresh the receipt at launch, quite often, I am prompted for the sign-in again. Is there a better way to handle the scenario in practice?

[hamzabinamin]

If the user cancels an auto renewable subscription from the settings of his phone, what would be the right field in the receipt result to check for that? is_cancelled?

[harryworld]

cancellation_date @hamzabinamin