lib/java TSimpleJSONProtocol can emit invalid JSON

TSimpleJSONProtocol can emit JSON with maps whose keys are not string (which is not allowed is the JSON spec).

This happens if the key in a map is anything other than a String (int, enum, etc)

For example, it can emit JSON like this:

{"myMap":{5:"myValue"}}

which should be:

{"myMap":{"5":"myValue"}}

I have a path that fixes this, I'll upload it shortly (still trying to get my dev environment to run the tests).

Also AFAICT there is no unit test for TSimpleJSONProtocol – I'll try and add one to the patch.

Thanks!
Alex