Add sandbox agnostic receipt verification

[ninjaboy]

As per Apple receipt verification best practices:

As a best practice, always call the production URL for verifyReceipt first, and proceed to verify with the sandbox URL if you receive a 21007 status code.

In case if isTest parameter is undefined or defined as null (basically means that developer prefers to perform environment agnostic check:

1. Query production verification endpoint
2. In case if 20017 error is returned - query sandbox receipt verification endpoint

For the cases when isTest parameter is explicitly defined as true or false - run the old logic

[hyochan]

@iaphub Could you kindly review this? Looks like this is somehow related to what you are working on.

[hyochan]

I've just gone through this and this is what Apple wanted. Thanks for your contribution 💯