Pass keyword args from `test_client` method to client class constructor

I believe this would be useful for customizing test clients. For example:

```python
class TestClient(FlaskClient):
    def __init__(self, authenticated=False, *args, **kwargs):
        self._authenticated = authenticated
        FlaskClient.__init__(*args, **kwargs)
```

And then I could say...

```python
client = app.test_client(authenticated=True)
```

`test_client()` can then act more as a factory.