TST: Fixed failures in JSON asserts
Fixes an occasional failure in the json tests. They'd fail when the
Series held objects of equal length.

```pytb
pandas/tests/extension/json/test_json.py:114:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas/util/testing.py:1224: in assert_series_equal
    _testing.assert_almost_equal(left.get_values(), right.get_values(),
pandas/core/series.py:466: in get_values
    return self._data.get_values()
pandas/core/internals.py:4742: in get_values
    return np.array(self._block.to_dense(), copy=False)
pandas/core/internals.py:1940: in to_dense
    return np.asarray(self.values)
../../Envs/pandas-dev/lib/python3.6/site-packages/numpy/numpy/core/numeric.py:500: in asarray
    return array(a, dtype, copy=False, order=order)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = {'a': 1}, key = 0

    def __getitem__(self, key):
        if key in self.data:
            return self.data[key]
        if hasattr(self.__class__, "__missing__"):
            return self.__class__.__missing__(self, key)
>       raise KeyError(key)
E       KeyError: 0
```

Work around that by converting to dicts before comparing.