Fix TypeError when event.target is HTMLDocument

[torlye]

If you put a ContextualMenu and a DatePicker on the same page; and click inside the DatePicker; you get an unhandled TypeError.

ContextualMenu _onDocumentClick asserts event.target as HTMLElement. However, when you click in the DatePicker's textfield; the DatePicker will fire an event with the document object as a target. (Jquery.DatePicker.ts line 112)

Since the document object is an HTMLDocument, not an HTMLElement, classList ends up as undefined. Hence you get the TypeError when trying to access classList.contains.

Attachment: A minimal HTML page to reproduce the problem. (Rename the file extension)

contextualmenu-datecontrol-bug.html.txt

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[Jahnp]

Hey @torlye, so sorry for the delay here, didn't see the notification for this PR. The changes look great--thank you! I'll go ahead and merge this in.