Fix: divider causes optGroup to become unchecked incorrectly

[jstrawther]

To reproduce...

1. Create a multiselect with multiple optgroups separated by dividers.
2. Select all options in an optgroup. Header is checked.
3. Call deselect method to deselect array of items by value from another optgroup.
4. The first optgroup is automatically deselected, even though all options are still selected.

Issue:
Examine the call to updateOptGroups on line 1369. When determining checked property of each optGroup, the divider option is included in the $options collection for consideration; but it should not be.