Backwards compatibility issue with JSONFormatter

[Dan Smith]

I think the changes for GEODE-6327 broke backwards compatibility in
JSONFormatter with the change from fromJSON(String jsonString) to
fromJSON(String jsonString, String... identityFields)

Adding an additional varargs parameter to the method breaks code that was
compiled against the non-varargs version. I think we need to overload the
method instead.

Thanks to Gester for discovering this with his test!

-Dan