Fix scrollBehavior='none' on path update

Fixes #369.

Test Plan: npm test

Author: sophiebits

modules/mixins/ScrollContext.js

@@ -76,10 +76,10 @@ var ScrollContext = {
 
   updateScroll: function (path, actionType) {
     var behavior = this.getScrollBehavior();
-    var position = this._scrollPositions[path];
 
-    if (behavior && position)
-      behavior.updateScrollPosition(position, actionType);
+    if (behavior != null && this._scrollPositions.hasOwnProperty(path)) {
+      behavior.updateScrollPosition(this._scrollPositions[path], actionType);
+    }
   },
 
   /**

modules/mixins/__tests__/ScrollContext-test.js

@@ -29,6 +29,12 @@ describe('ScrollContext', function () {
     it('has a null scroll behavior', function () {
       expect(component.getScrollBehavior()).toBe(null);
     });
+
+    it('does not throw when updating scroll position', function () {
+      expect(function() {
+        component.updateScroll('/');
+      }).toNotThrow();
+    });
   });
 
   describe('when using scrollBehavior="browser"', function () {