* made a few attributes that aren't present in SKTest receipts optional, added test with IAP
* added more tests for every field in the receipt
* fixed a few places where `asDict` wasn't correctly considering nullability in the IAP receipt fields
* fixed length calculation for indefinite-length containers
* temporarily removed the early return from empty receipts
* fixed length calculation when parsing receipts with indefinite length
* fix linting
* added basic ugly test
* fixed local receipt parsing for sktest, broke regular receipt parsing temporarily
* updated length calculation so that it works for both storekit test and regular receipts
* updated test to allow for missing OriginalAppVersion in receipt
* fixed linting in imports
* fixed checks for non-storekitTest receipts
* added a longer comment to explain the length == 0 case
* cleaned up the still-ugly tests for the receipt parser
aboedo