Fixes broken test case.

t/unit/app/test_defaults.py

@@ -58,7 +58,7 @@ class test_defaults:
         find = self.defaults.find
 
         assert find('default_queue')[2].default == 'celery'
-        assert find('task_default_exchange')[2] == 'celery'
+        assert find('task_default_exchange')[2] is None
 
     @property
     def defaults(self):