`open` attribute should not be removed from `details` element

[alan-isaac]

Summary: open attribute is incorrectly removed from details element

Steps to reproduce:

1. edit a Canvas page, switch to HTML editor and create a details element with open attribute
2. save the changes
3. again open the editor, and examine the results

Expected behavior: open attribute still present

Actual behavior: open attribute missing (i.e., it was sanitized)

[github-actions]

Thanks for contributing to this issue. As it has been 2 years since the last activity, we are automatically closing the issue in 30 days. This is often because the request was already solved in some way and it just wasn't updated or it's no longer applicable. If that's not the case, please respond before the issue is closed, or post a message on the mailing list. We'll gladly take a look again!

[alan-isaac]

This issue remains, exactly as described.