Gated datasets without authentication header return 404

[severo]

It should return 401 (Unauthorized).

See for example https://datasets-server.huggingface.co/splits?dataset=severo/bigcode/the-stack from a private window.

Or https://datasets-server.huggingface.co/splits?dataset=JosephusCheung/GuanacoDataset (if you have not access to it), while passing credentials (opening it while logged in Hugging Face will pass your cookie)

[severo]

When processing this issue, try to also test authenticated against authorized, and add examples in the openapi spec if needed