Fix failing Backbone test and ensure _.assign works properly with a customizer that returns undefined.

lodash.js

@@ -1694,11 +1694,12 @@
         if (customizer) {
           var value = object[key],
               result = customizer(value, source[key], key, object, source);
+
+          if (result !== value || (typeof value == 'undefined' && !(key in object))) {
+            object[key] = result;
+          }
         } else {
-          result = source[key];
-        }
-        if (!(customizer && result === value)) {
-          object[key] = result;
+          object[key] = source[key];
         }
       }
       return object;

test/test.js

@@ -953,6 +953,11 @@
       deepEqual(actual, { 'a': 1, 'b': 2, 'c': 3 });
     });
 
+    test('should work with a `customizer` that returns `undefined`', 1, function() {
+      var expected = { 'a': undefined };
+      deepEqual(_.assign({}, expected, _.identity), expected);
+    });
+
     test('should be aliased', 1, function() {
       strictEqual(_.extend, _.assign);
     });