Port bugfix from jupyter_server

[kycutler]

jupyter-server/jupyter_server#628 fixed an issue in jupyter_server, but was never ported over to the notebook backend. I'm not sure what the standard procedure here is, so I'm opening this issue to hopefully get the ball rolling as this would be great to have fixed in both backends if possible.

[JasonWeill]

@blink1073 made the change in jupyter-server/jupyter_server#628 — should this be ported into this package as well?

[blink1073]

I think so, but you'd also need jupyter-server/jupyter_server#636 to account for kernels that don't expect that message on the control channel.

[kycutler]

@blink1073 Would you mind reviewing the PR I opened to close this?