fix issue #437

```
import json
queue.enqueue(json.dumps, '1', description='中文')
```
now it works fine.

```
18:50:20 RQ worker started, version 0.4.6
18:50:20
18:50:20 *** Listening on default...
18:50:24 default: 中文 (1bb4dab7-8525-43d1-8bfe-2761d2080ce7)
18:50:24 Job OK, result = "1"
18:50:24 Result is kept for 500 seconds.
18:50:24
18:50:24 *** Listening on default...
```