Renewed update to work with spaces in categories

I updated `postfiles.rb` to work with posts that have a space in the category. Unlike the previous contributor, I noticed that spaces (and I assume other special characters) got escaped by Jekyll before storing in the `post.id`. Did not check what caused this change in behaviour, assuming something changed in jekyll.

Tested it only on a small site of my own (not an experienced Ruby programmer yet), so please review carefully.