Fix _.extend regression in IE < 9.

Former-commit-id: 65632fbbf3aaba0d9cd8578300f8025e311faff0

test/backbone.html

@@ -5,7 +5,10 @@
 		<title>Backbone Test Suite</title>
 		<link rel="stylesheet" href="../vendor/backbone/test/vendor/qunit.css">
 		<style>
-			body > #qunit-header, #jslitmus {
+			body > #qunit-header {
+				display: none;
+			}
+			#jslitmus {
 				display: none;
 			}
 		</style>

test/test.js

@@ -149,6 +149,14 @@
   QUnit.module('lodash.extend');
 
   (function() {
+    test('should not error on `null` or `undefined` sources (test in IE < 9)', function() {
+      try {
+        deepEqual(_.extend({}, null, undefined, { 'a': 1 }), { 'a': 1 });
+      } catch(e) {
+        ok(false);
+      }
+    });
+
     test('skips the prototype property of functions (test in Firefox < 3.6, Opera > 9.50 - Opera < 11.60, and Safari < 5.1)', function() {
       function Foo() {}
       Foo.prototype.c = 3;