`fenced_divs` doesn't support unbraced class names in commonmark

**Explain the problem.**
```
$ printf ':::test\n:::' | pandoc -f markdown+fenced_divs
<div class="test">

</div>
$ printf ':::test\n:::' | pandoc -f commonmark+fenced_divs
<p>:::test :::</p>
```

**Pandoc version?**
pandoc 2.16.2 on Linux
