Result: .forget() should also clear local cache

celery/result.py

@@ -96,6 +96,7 @@ class AsyncResult(ResultBase):
 
     def forget(self):
         """Forget about (and possibly remove the result of) this task."""
+        self._cache = None
         self.backend.forget(self.id)
 
     def revoke(self, connection=None, terminate=False, signal=None,