Task decorator should not be lazy if app already finalized. Closes #1571

celery/app/base.py

@@ -198,7 +198,7 @@ class Celery(object):
                         task = filter(task)
                     return task
 
-                if opts.get('_force_evaluate'):
+                if self.finalized or opts.get('_force_evaluate'):
                     ret = self._task_from_fun(fun, **opts)
                 else:
                     # return a proxy object that evaluates on first use