Add subclass test object to showcase inheritance problems

I am suffering from [the issue reported here](https://github.com/bradjasper/django-jsonfield/issues/51). Your latest commits are related to this as well. I've modified the tests to show the issue, but have not found the way to 'fix' it yet. The errors look like this now:
```
jsonfield/tests.py", line 67, in test_string_in_json_field
    self.assertEqual(new_obj.json, json_obj)
AssertionError: u'"blah blah"' != 'blah blah'
```