Fix an issue with <ol>s nested inside <ul>s.

If you have an `<ol>` tag nested inside a `<ul>` tag, the `<ol>` tag will have bullets instead of numbers. This fixes that by resetting `<ol>`'s list image to "none".

I encountered this bug at a tweaked version of this theme that I'm using at http://haikuarchives.github.io/uploadrepo.html.