ContextualMenu: Default not prevented after handling keydown

[T-Hugs]

Bug Report

• Package version(s): (fill this out)
• Browser and OS versions: All

Describe the issue:

ContextualMenu: Default not prevented after handling keydown

Actual behavior:

Expected behavior:

Default is prevented so the parent doesn't decide to handle the event.

If applicable, please provide a codepen repro:

[T-Hugs]

I was going to submit a pull request, but I don't have time to complete the entire checklist. In ContextualMenu.tsx, the line ev.preventDefault(); should be included in the _onItemKeyDown method, inside the if-statement.

[dzearing]

@trevorsg if you could submit a PR, we can take a look and help with the missing checkboxes. @joschect FYI

[dzearing]

Merged, published tomorrow.