Fix `clean_path_info` for paths with only a slash

When using `Rack::Static` to serve from the root and requesting the root resource, it attempts to `clean_path_info("/")` which results in an exception because the `clean` array is empty.

This fixes it by prepending the slash if `clean` is empty or the first element is empty.
nit: this test is misnamed. The "clean" occurs, it just doesn't fail. Maybe "should clean slash only paths".