Use category in downcase only for URL

Resolves #1739.
It preserves letter case of category, but return `category.downcase` for `url_placeholders`. Also added some proper tests. If there are needed updates for documentation, please let me know.
This will have to be `uniq`'d as well. If I have `Movies` and `movies`, I don't want `/movies/movies/2014/07/06/my-post.html`.