reverting an old change -- _.extend should absolutely copy undefined values.

test/objects.js

@@ -38,7 +38,7 @@ $(document).ready(function() {
     result = _.extend({x:'x'}, {a:'a', x:2}, {a:'b'});
     ok(_.isEqual(result, {x:2, a:'b'}), 'extending from multiple source objects last property trumps');
     result = _.extend({}, {a: void 0, b: null});
-    equals(_.keys(result).join(''), 'b', 'extend does not copy undefined values');
+    equals(_.keys(result).join(''), 'ab', 'extend does not copy undefined values');
   });
 
   test("objects: defaults", function() {

underscore.js

@@ -635,7 +635,7 @@
   _.extend = function(obj) {
     each(slice.call(arguments, 1), function(source) {
       for (var prop in source) {
-        if (source[prop] !== void 0) obj[prop] = source[prop];
+        obj[prop] = source[prop];
       }
     });
     return obj;