Preserve quotes if attribute ends with a trailing slash

minified URLs in parameters, like <a href="http://www.example.com/"> need to keept their quotes because <a href=http://www.example.com/> is interpreted as an URL without trailing slash
Added a test that fails if attribute quotes are stripped with a trailing slash
Added a comment to make the reason behind the expression more verbose
Merge pull request #93 from stieler-it/patch-1

Preserve quotes if attribute ends with a trailing slash