Undo Markdown formatting by applying format again

[ergl]

I expected that, with a word selected, applying styling twice would remove the format. Instead, it seems to style it again. Here's what I mean:

• Select word foo, press Cmd+B. It becomes **foo**
• Select it again, either the foo part, or the entire **foo** fragment, and press Cmd+B again.
• The result is ****foo****, while I expected it to go back to foo again (this is also what Github does).

The same happens with italics or strikethrough.

EDIT: I see this is already the default behaviour when editing normal text cells. Would be nice to apply it to markdown cells too.