Fix range slider with label

This fixes the ability to update a selector with the value of the range slider, introduced in <a href="https://github.com/zurb/foundation/commit/e73182e35475fdda0c5f146b64c9042643550087"> e7318</a> and broken in <a href="https://github.com/zurb/foundation/commit/761422937734abec3475a8021e4f8dbecda7e33f">76142</a>.

I also added a rudimentary test.