uib-uncheckable doesn't obey booleans

[Nokel81]

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

[zacronos]

I'm surprised this has sat (with a PR) for nearly a month without even any comment -- this is not an edge case bug, the UIB Getting Started page even shows this bug.

[Nokel81]

Same, it is quite annoying