When HTML_SANITIZE_OUTPUT is turned on, self-closing tags like `<hr />` become rewritten as `<hr>`. I know it's still valid HTML, but there's no reason to be changing them.

It can be fixed either by handling the `html.SelfClosingTagToken` case via the same path as `html.EndTagToken` on line 100, or by adding an if statement on line 96 that does `wr.WriteString(" />")` when `t == html.SelfClosingTagToken`.