/_debugtoolbar/ is a 404 until request hit the main app

To reproduce:

1. Start your Pyramid app (my listens on port 5000).
1. Request ``http://localhost:5000/_debugtoolbar/``. This will produce a 404 error.
1. Request any URL outside the debug toolbar.
1. Request ``http://localhost:5000/_debugtoolbar/`` again. You will now see the debug toolbar.