fix(buttons): fix uncheckable attribute

- Fix behavior of `uib-uncheckable` to agree with intention

Closes #5451
Fixes #5412

Author: c-vetter

Diff for: src/buttons/buttons.js

@@ -44,7 +44,7 @@ angular.module('ui.bootstrap.buttons', [])
 
       if (attrs.uibUncheckable) {
         scope.$watch(uncheckableExpr, function(uncheckable) {
-          attrs.$set('uncheckable', uncheckable ? '' : null);
+          attrs.$set('uncheckable', uncheckable ? '' : undefined);
         });
       }
     }

Diff for: src/buttons/test/buttons.spec.js

@@ -328,12 +328,38 @@ describe('buttons', function() {
         $scope.uncheckable = false;
         var btns = compileButtons('<button ng-model="model" uib-btn-radio="1">click1</button><button ng-model="model" uib-btn-radio="2" uib-uncheckable="uncheckable">click2</button>', $scope);
         expect(btns.eq(0).attr('uncheckable')).toBeUndefined();
-        expect(btns.eq(0).attr('uncheckable')).toBeUndefined();
+        expect(btns.eq(1).attr('uncheckable')).toBeUndefined();
+
+        expect($scope.model).toBeUndefined();
+
+        btns.eq(0).click();
+        expect($scope.model).toEqual(1);
+
+        btns.eq(0).click();
+        expect($scope.model).toEqual(1);
+
+        btns.eq(1).click();
+        expect($scope.model).toEqual(2);
+
+        btns.eq(1).click();
+        expect($scope.model).toEqual(2);
 
         $scope.uncheckable = true;
         $scope.$digest();
         expect(btns.eq(0).attr('uncheckable')).toBeUndefined();
         expect(btns.eq(1).attr('uncheckable')).toBeDefined();
+
+        btns.eq(0).click();
+        expect($scope.model).toEqual(1);
+
+        btns.eq(0).click();
+        expect($scope.model).toEqual(1);
+
+        btns.eq(1).click();
+        expect($scope.model).toEqual(2);
+
+        btns.eq(1).click();
+        expect($scope.model).toBeNull();
       });
     });
   });