Highlighting broken with multiple: true and closeOnSelect: false

The index of the current choice being moused-over was being passed into highlight.  However, in `highlightUnderEvent`, the index was being calculated against all .select2-result choices, which included selected options.  In `highlight`, the index was being calculated against only visible elements.  

An alternate fix might be:
`var choices = this.results.find('.select2-result:visible');` 
since this seems to duplicate the logic in `highlight`.  

The symptoms for this seemed similar to #348, but that fix is still on master; I think the implementation of `highlight` has evolved since that fix, which may have caused the issue to resurface.