Removing active class when selected value was removed

[izabellaszlosarczyk]

Bug description:

Following scenario:

1. Select one option from ui-select
2. Click button (remove value)
3. Toggle select
   You can see that previously selected option has still "active" class - when other stuff (like placeholder) were reset to state before selecting.
   When $select.selected is empty (undefined,empty string,null) active class should be removed from choice too (I am setting $select.activeIndex = -1).

Link to minimally-working plunker that reproduces the issue:

https://codepen.io/anon/pen/YJXYKL

Version of Angular, UI-Select, and Bootstrap/Select2/Selectize CSS

Angular:
1.6.5
UI-Select:
latest
Bootstrap/Select2/Selectize CSS (if applicable):
3.1.1