Dropkick does not correctly handle selects with optgroups

[adamgruber]

Version of Dropkick:

2.2.5 (Observed in prior versions as well)

Expected Behavior

When opening the dropdown, the selected item should be in view.

Actual Behavior

When <optgroup>s are used, the selected item may not appear in view when the dropdown is opened.

Steps To Reproduce:

1. Run dropkick on a <select> that uses one or more <optgroup>s
2. Select an item in the list
3. Open the dropdown again and observe the selected item is not in view

JSBin / Example repo / Failing test:

https://github.com/adamgruber/dropkick-optgroup-issue