Mouse does not change active item

[mkresse]

Bug description:

In the current version of the select dropdown, two different highlights are used at the same time. For example with the Select2 theme, the row below the mouse is highlighted with a gray background using the css :hover, while the active item for usage with the cursor keys is highlighted with a blue background. It is confusing for the user to understand why the first row is marked blue and doesn't change, even if the mouse is moved across other items, as for example noted in #2039 .
The original Select2 nor plain HTML suffer from that issue and have only one type of highlighting.

Link to minimally-working plunker that reproduces the issue:

http://next.plnkr.co/edit/BUrB9eM7qcDAHYnc (based on "Basic demo")

The blue and gray highlight are independent from each other, mouse movement does not affect the heavy highlight.

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

Angular: 1.5

UI-Select: 0.19.8

Bootstrap/Select2/Selectize CSS (if applicable): 3.1.1 / 3.4.5