Fix for JCLOUDS-655

First time committing to jclouds, let me know if there are any style guidelines I missed with this change.
D'oh! Good point about the additional unit test for the JsonObject case, I'll get that added ASAP. I'm not sure about testing the class directly though, I'd actually considered doing that originally but the other adapters didn't have any unit tests. I figured that would have just been overkill, but I'm happy to add one if you guys think it's necessary.