Cannot set OriginalTransactionID in ReceiptForIOS6 type using same field from InApp due to type mismatch

[austinhess]

Hello, we are trying to use fields from a Receipt and an InApp to create a ReceiptForIOS6 . We are attempting to set the ReceiptForIOS6 based on latestReceipt *InApp as follows:

ReceiptForIOS6{
    ... setting other fields
    OriginalTransactionID: latestReceipt.OriginalTransactionID,
    ...
}

but get a type mismatch error, as latestReceipt.OriginalTransactionID is a string, but a numericString is provided by the InApp object. Because numericString is a private type, we cannot cast to it or otherwise generate a new numericString (even with proper validation). To address this issue previously, we have forked this repo to make numericString a public type, but would prefer to use this main repo so as to not miss out on important updates. We would hope that one of the following options would be acceptable to resolve this issue, and if so, we can submit a PR to do so:

• Making numericString a public type NumericString
• Making a new public function, such as GetNumericStringFromString(input string) (numericString, error) that allows us to get an instance of this type, possibly subject to the same validation as in the UnmarshalJSON function
• If the ReceiptForIOS6 OriginalTransactionID field type is unnecessarily specific, it could be changed to a string type, or if the InApp type should be requiring a numeric string for that field, that field could be updated to be a numericString.

We would prefer the first option. This issue may be related to #160, but more details would be needed.