**Hello&#32;** doesn't roundtrip via CommonMark

See https://github.com/gjtorikian/commonmarker/issues/363:

```ruby
markdown = '**Hello&#32;**'
doc = Commonmarker.parse(markdown)
puts doc.to_commonmark # outputs the wrong thing: **Hello **
```

Note present in upstream:

```console
$ cmark --version
cmark 0.31.1 - CommonMark converter
(C) 2014-2016 John MacFarlane
$ echo '**Hello&#32;**' | ./cmark -t commonmark
**Hello **
```