fix:jqLite: Normalize non-existing attributes to undefined as jQuery

jqLite was returning null, but jQuery returns undefined

src/jqLite.js

@@ -266,7 +266,9 @@ forEach({
     } else if (element.getAttribute) {
       // the extra argument "2" is to get the right thing for a.href in IE, see jQuery code
       // some elements (e.g. Document) don't have get attribute, so return undefined
-      return element.getAttribute(name, 2);
+      var ret = element.getAttribute(name, 2);
+      // normalize non-existing attributes to undefined (as jQuery)
+      return ret === null ? undefined : ret;
     }
   },
 

test/jqLiteSpec.js

@@ -162,6 +162,10 @@ describe('jqLite', function(){
       expect(select.attr('multiple')).toEqual(true);
     });
 
+    it('should return undefined for non-existing attributes', function() {
+      var elm = jqLite('<div class="any">a</div>');
+      expect(elm.attr('non-existing')).toBeUndefined();
+    });
   });