fix(buttons): allow selection of undisabled button

- When a button has been toggled between disabled and not, fix allowing selection of the button to occur

Fixes #4088

src/buttons/buttons.js

@@ -24,7 +24,7 @@ angular.module('ui.bootstrap.buttons', [])
 
       //ui->model
       element.bind(buttonsCtrl.toggleEvent, function () {
-        if ('disabled' in attrs) {
+        if (attrs.disabled) {
           return;
         }