1. Update the test app
If I understand correctly, the debug toolbar will only be enabled if `app.debug` is `True`. So I added the `DEBUG=True` to the test app.
Related code: 2b8bf9cc44/src/flask_debugtoolbar/__init__.py (L114)
2. Update the `src/flask_debugtoolbar/__init__.py`
Fix the two if statements to prevent the following errors:
```
> if 'gzip' in response.headers.get('Content-Encoding'):
E TypeError: argument of type 'NoneType' is not iterable
```
Since the `response.headers.get('Content-Encoding')` could be None.
With this PR, all the tests will be passed. The failed style checker will be fixed in #219
When I switched over to `flask run` entrypoint in b92391d177,
I forgot that the `if name==__main__` code no longer triggers.
So the SQLite in-memory database wasn't getting created for the example
app.
This moves the DB creation to a werkzeug/Flask hook that runs before the
first request to the app, so that the DB table is created when we query
it.
Also updated the test which worked fine previously, but this is more
idiomatic.
Drop `flask_script` in favor of Flask's native CLI:
* https://flask.palletsprojects.com/en/master/cli/
This also requires changing the tests so that `pytest` mocks the env var
`FLASK_ENV` so that the test app starts in development mode. Unlike
normal test apps, we _do_ want development/debug mode, in addition to
testing mode.
