fix Cassandra backend TTLs

celery/backends/cassandra.py

@@ -142,11 +142,11 @@ class CassandraBackend(BaseDictBackend):
             if self.detailed_mode:
                 meta['result'] = result
                 cf.insert(task_id, {date_done: self.encode(meta)},
-                          ttl=timedelta_seconds(self.expires))
+                          ttl=self.expires and timedelta_seconds(self.expires))
             else:
                 meta['result'] = self.encode(result)
                 cf.insert(task_id, meta,
-                          ttl=timedelta_seconds(self.expires))
+                          ttl=self.expires and timedelta_seconds(self.expires))
 
         return self._retry_on_error(_do_store)