Disabled optgroup not detected

[Benjamin Milde]

Version of Dropkick:

2.1.8

Expected Behavior

Optgroup items and it's label should be marked disabled.

Actual Behavior

Both work as if they were not disabled. There are also no classes about it added to the markup.

JSBin / Example repo / Failing test:

Here is a codepen template for reproducing bugs: http://codepen.io/LostKobrakai/pen/XKdbEK