do not remove suffix if a forward slash

If a site is configured with `$config['uri_suffix'] = '/';`, the new URI::_remove_url_suffix() mistakenly removes the last separating slash. This fix bypasses the remove if the url_suffix config is a slash.

This config value may be sent mistakenly or intentionally, but would cause site breakage by this function because the last separating slash would be removed.

`blog/post/34` would become `blog/post34` and would result in a 404.