Use correct plain string for conversion tests
Use correct plain string for conversion tests

Pandoc has [changed the format of plain output]: headings are
capitalized and set off from the text with spacing, emphasized text gets
wrapped in markdown's `_`.

[1]: https://github.com/jgm/pandoc/commit/3eff3782c1cfde8c1cdb4d5461dfb4bf09168e9c

dalehamel/ruby-pandoc,alphabetum/pandoc-ruby,alphabetum/pandoc-ruby