Remove user after login cookie cleared

Signed-off-by: Aaron Huang <aaroms9733@gmail.com>

jupyterhub/handlers/base.py

@@ -484,6 +484,8 @@ class BaseHandler(RequestHandler):
             path=url_path_join(self.base_url, 'services'),
             **kwargs
         )
+        # Reset _jupyterhub_user
+        self._jupyterhub_user = None
 
     def _set_cookie(self, key, value, encrypted=True, **overrides):
         """Setting any cookie should go through here