transaction.Timestamp:toString() sometimes throws ClassCastException

[gimantha]

Please check the line https://github.com/ballerina-platform/module-ballerinai-transaction/blob/master/transaction-native/src/main/java/org/ballerinalang/stdlib/transaction/ToString.java#L36

The nativeDataObject here is sometime a long sometime it is an integer. This causes the tests fail. As a temporary fix I used Number to get the long value