Fix deprecated FlagConditionsMiddleware NOOP

This change restores response generation to the FlagConditionsMiddleware instead of just `pass`ing. This is so that when the middleware is in-use it doesn't interrupt the process of sending the request through all of the configured middleware.

flags/middleware.py

@@ -8,6 +8,7 @@ def __init__(self, get_response):
             "effect. It will be removed in a future version of Django-Flags. ",
             FutureWarning,
         )
+        self.get_response = get_response
 
     def __call__(self, request):
-        pass
+        return self.get_response(request)