Headings conversion issue

This seems to be a bug

```
puts Kramdown::Document.new("# aaa\n# bbb").to_html
# <h1 id="aaa">aaa</h1>
# <p># bbb</p>

puts Kramdown::Document.new("# aaa\n\n# bbb").to_html
# <h1 id="aaa">aaa</h1>
#
# <h1 id="bbb">bbb</h1>
```

CommonMark spec says blank lines are not required: http://spec.commonmark.org/0.27/#example-47