Fix the redirect handler

[jtpio]

Fixes #6940

@balajialg trying locally with jupyterhub with the following config, it looks like this should fix the issue

c = get_config()  # noqa

from jupyterhub.auth import DummyAuthenticator

c.JupyterHub.authenticator_class = DummyAuthenticator

from jupyterhub.spawner import SimpleLocalProcessSpawner

c.JupyterHub.spawner_class = SimpleLocalProcessSpawner
c.JupyterHub.bind_url = 'http://0.0.0.0:8000'
c.Spawner.cmd = ["jupyter-labhub"]
c.Spawner.default_url = '/tree'

Before

jupyterhub-redirect-issue.mp4

After

jupyterhub-redirect.mp4

[github-actions]

👈 Launch a Binder on branch jtpio/notebook/redirects

[jtpio]

Getting that one in and following-up with a release since it seems to be fixing #6940