make sure celery app is not patched with ProxyFix (see #311)

burpui/engines/worker.py

@@ -20,7 +20,7 @@ sys.path.insert(0, os.path.join(os.path.dirname(os.path.realpath(__file__)), '..
 def init_app(conf):
     from burpui import create_app
     from burpui.ext.tasks import celery
-    app = create_app(conf, celery_worker=True)
+    app = create_app(conf, celery_worker=True, gunicorn=False)
     return app, celery