Extends default `response_class` to test JSON responses

Add `json` method/property to `application.response_class` like the `requests` library
or `Flask-Testing` does:

```python

def test_json_response(client, accept_json):
    res = client.get(url_for('api.ping'), headers=accept_json)
    assert res.json == {'ping': 'pong'}
```