improve #isEmpty implementation. add 2 tests

test/objects.js

@@ -60,6 +60,8 @@ $(document).ready(function() {
     ok(_.isEmpty([]), '[] is empty');
     ok(!_.isEmpty({one : 1}), '{one : 1} is not empty');
     ok(_.isEmpty({}), '{} is empty');
+    ok(_.isEmpty(null), 'null is empty');
+    ok(_.isEmpty(), 'undefined is empty');
 
     var obj = {one : 1};
     delete obj.one;

underscore.js

@@ -503,7 +503,9 @@
 
   // Is a given array or object empty?
   _.isEmpty = function(obj) {
-    return _.keys(obj).length == 0;
+    if (_.isArray(obj)) return obj.length===0;
+    for (var k in obj) return false;
+    return true;
   };
 
   // Is a given value a DOM element?