**Hello ** doesn't roundtrip via CommonMark

[kivikakk]

See gjtorikian/commonmarker#363:

markdown = '**Hello **'
doc = Commonmarker.parse(markdown)
puts doc.to_commonmark # outputs the wrong thing: **Hello **

Note present in upstream:

$ cmark --version
cmark 0.31.1 - CommonMark converter
(C) 2014-2016 John MacFarlane
$ echo '**Hello **' | ./cmark -t commonmark
**Hello **

[kivikakk]

Rather than try to generically fix All Of CommonMark Output, let's fix this problem: spaces close to delimiters should be entity-encoded according to their flankingness.