fetchReceipt: replace String with Data

[bizz84]

The receipt read from Bundle.main.appStoreReceiptURL is a Data object.

Maybe it makes sense to return it as such and let clients convert it into a string if needed (for example when uploading as payload to a server):

let receipt = receiptData.base64EncodedString(options: [])

Also, local receipt verification may access the bytes from Data rather than String.