Fix matching of void elements to always match full tag name

HTML parser matches `colgroup` as a void element and then throws _RuntimeError: Outdented too far on line X_. The problem is in [html_parser.rb:29](https://github.com/threedaymonk/htmlbeautifier/blob/master/lib/htmlbeautifier/html_parser.rb#L29) – see [online example](http://rubular.com/r/1YMghmeMa9).

I’ve fixed this issue and added regression and integration tests.