uib-uncheckable doesn't obey booleans #6532

Bug description:

The uib-uncheckable tag does not seem to work with setting boolean variables

Link to minimally-working plunker that reproduces the issue:

https://plnkr.co/edit/JcOeDtnONqyzJdCLrkqk (The example one for uib-btn-radio on the github website)

Steps to reproduce the issue:

Click onto add off of the 'Right' radio button while also clicking the 'Toggle Uncheckable' button. The value of 'uncheckable' is displayed for convenience.

Version of Angular, UIBS, and Bootstrap

Angular: 1.6.1

UIBS: 2.5.0

Bootstrap: 3.3.7