more robust `U.insert_html_class`

Just handling a few more edge cases in this patch - specifically
when the current is nil and the ones to insert have duplicates.

/cc @jcredding.  just a patch.
version to 0.4.1

* more robust `U.insert_html_class` (55f88a62247b34d06d37af68484df82a6d225e84)