Yicheng Qin 2e41b4f9e1 etcdserver/auth: fix return value when creating root user
Before:

```
$ curl http://127.0.0.1:4001/v2/auth/users/root -XPUT -d '{"user": "root",
"password": "root"}'
{"user":"root","roles":null}
```

After:

```
{"user":"root","roles":["root"]}
```
2015-06-27 23:16:54 -07:00
..
2015-06-15 18:18:50 -04:00
2015-06-25 14:25:08 -04:00