Navigation keys in data array break theme_item_list()

The first and last classes from theme_item_list() are damaged by the keys passed in theme_multistep_navigation(). I haven't tested in D7 yet, but the same key structure is passed there too.

I understand these keys were introduced for sorting, but we can do it another way. The attached patch doesn't touch the theme function (especially because this is likely already overridden on existing sites), but sorts then cleans the array keys before passing to the theme layer.