Run pre_save_hook before model check

[davidbrochart]

The pre_save_hook can potentially change the model, so checking for the type and content should be done after the call to the hook.
This will be needed in JupyterLab when we save documents from the back-end. In this case, the content won't be sent through the HTTP endpoint, but the hook will populate the model's content from the RTC server.

See jupyterlab/jupyterlab#11599.

[codecov-commenter]

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[blink1073]

LGTM, thanks!

[Zsailer]

Thanks, @davidbrochart. I don't see a major risk in moving the model checks after the pre_save_hook.