Fix bug with expired session

If the session is expired the Redis driver try to set $this->keys['previous_id'] with $this->keys['session_id'] which do not exist also it try to set $this->keys['session_id']  with $payload['rotated_session_id'] but we have just set the payload variable with the retated_session from Redis.
We just need to unserialize the data like in Memcached and File driver.
Merge pull request #1433 from daidais/patch-1

Fix bug with expired session