fix(ngModelOptions): enable overriding the default with a debounce of zero

Because of how the logic was set up, a value of `0` was assumed to be the
same as `undefined`, which meant that you couldn't override the default
debounce delay with a value of zero.

For example, the following assigned a debounce delay of 500ms to the `blur`
event.

```
ngModelOptions="{ updateOn: 'default blur', debounce: {'default': 500, 'blur':
0} }"
```

Closes angular#7205

Author: joelhooks

src/ng/directive/input.js

@@ -1822,9 +1822,20 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * @param {string} trigger Event that triggered the update.
    */
   this.$setViewValue = function(value, trigger) {
-    var debounceDelay = ctrl.$options && (isObject(ctrl.$options.debounce)
-        ? (ctrl.$options.debounce[trigger] || ctrl.$options.debounce['default'] || 0)
-        : ctrl.$options.debounce) || 0;
+    var debounceDelay = 0,
+        options = ctrl.$options,
+        debounce;
+
+    if(options && isDefined(options.debounce)) {
+      debounce = options.debounce;
+      if(isNumber(debounce)) {
+        debounceDelay = debounce;
+      } else if(isNumber(debounce[trigger])) {
+        debounceDelay = debounce[trigger];
+      } else if (isNumber(debounce['default'])) {
+        debounceDelay = debounce['default'];
+      }
+    }
 
     $timeout.cancel(pendingDebounce);
     if (debounceDelay) {

test/ng/directive/inputSpec.js

@@ -858,6 +858,25 @@ describe('input', function() {
       expect(scope.checkbox).toBe(false);
     }));
 
+    it('should allow selecting 0 for non-default debounce timeouts for each event on checkboxes', inject(function($timeout) {
+      compileInput('<input type="checkbox" ng-model="checkbox" '+
+        'ng-model-options="{ '+
+          'updateOn: \'default blur\', debounce: { default: 10000, blur: 0 } }"'+
+        '/>');
+
+      inputElm[0].checked = false;
+      browserTrigger(inputElm, 'click');
+      expect(scope.checkbox).toBe(undefined);
+      $timeout.flush(8000);
+      expect(scope.checkbox).toBe(undefined);
+      $timeout.flush(3000);
+      expect(scope.checkbox).toBe(true);
+      inputElm[0].checked = true;
+      browserTrigger(inputElm, 'click');
+      browserTrigger(inputElm, 'blur');
+      $timeout.flush(0);
+      expect(scope.checkbox).toBe(false);
+    }));
 
     it('should inherit model update settings from ancestor elements', inject(function($timeout) {
       var doc = $compile(