fix(button): convert exotic model values to boolean view value

Fix for AngularJS 1.3 breaking changes when using exotic values for checkbox. The view value for the input type checkbox needs to be a boolean?

This seems to be related to this commit:

https://github.com/angular/angular.js/commit/c90cefe16142d973a123e945fc9058e8a874c357

and changes to the ngTrueValue/ngFalseValue attributes behaviour.