exempt_routes gets appended with auth disabled route on every call

[volinthius]

In FalconAuthMiddleware the exempt_routes list gets appended with auth disabled route on every call to a resource which has auth_disabled set:

    def _get_auth_settings(self, req, resource):
        ...
        if auth_settings.get('auth_disabled'):
            auth_settings['exempt_routes'].append(req.uri_template)
        ...

This causes the list to grow indefinitely.