Ensure that PUT requests have some body
Ensure that PUT requests have some body

This happens because `nbserverproxy` sets the body to `None` for PUT requests with an empty body

https://github.com/jupyterhub/nbserverproxy/blob/1e0ce336370cde5f622ba38618eac25d1f8af9c7/nbserverproxy/handlers.py#L223

If a PUT request has `body==None` then Tornado complains with
message "Body must not be None for method PUT (unless allow_nonstandard_methods is true)"

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