fix(uiSelectHeaderGroupSelectableDirective): do not issue error if feature is disabled; apply selectable feature after options list changed/filtered; do not select unnamed group

[Klinton90]

Trying to use new feature on my project. Would like to fix issues as soon as I find it.

1. We use wrapper around ui-select, so we always need uiSelectHeaderGroupSelectableDirective directive to exist on template even when ui-select is used for single/no-groups mode. In current implementation, if directive is disabled and other conditions do not pass, console.error() will be executed. That fix prevents error message to be shown for disabled directive.

2. If list of available options is filtered on the fly (based on $select.search, e.g. ng-repeat="options | filter: $select.search"), not all group headers receive ui-select-header-group-selectable class, as DOM is updated after enableClick() function runs.

3. Do not add click listener to unnamed group

[DarkIsDude]

Hi @Klinton90 , look good to me 👍 .