Empty 'ended_at' value

When doing
```python
job = Job.fetch(job_id, connection=Redis())
print job.ended_at
```
it prints `None`.

There is a `result` and `status` is `finished`.

I also checked with the redis-cli to confirm the `ended_at` field doesn't exists.

I am using 0.3.2 but checked with master (0.3.3-dev)