Options binding should not cause a needless browser reflow

In an effort to keep the user's place when updating a multiple `<select>` element's options, the options binding caches its `scrollTop`, redraws the options, then restores the `scrollTop`. Unfortunately, querying the element's `scrollTop` [forces an expensive, synchronous reflow](http://calendar.perfplanet.com/2009/rendering-repaint-reflow-relayout-restyle/).

This patch avoids querying scrollTop if:

1. we're not dealing with a multiple `<select>`
2. we didn't have any options to start with