Update Json.java

EDIT: 
This request throws errors,
new pullrequest:
https://github.com/Phibedy/libgdx/commit/959516fe9035bb2f06da8d3633a799989edbcfc7

Added line 65-80 so it should be possible to use code like:
{
Json json = new Json()
json.initialzeJsonWriter(...)
json.writeObjectStart("ActionName");
json.writeValue("action", action, action.getClass());
json.writeObjectEnd();
}