back to unicode in decode_config

I fixed the code to work like
```
client.update_configuration({"mstr_": u"いろは"} )
config = client.get_configuration(timeout=5)
self.assertEqual(u"いろは", config['mstr_'])
```
 if you set reconfigure param in unicode, then you'll get them using unicode
see 
https://github.com/ros/dynamic_reconfigure/pull/45/files#diff-326aeceb164c22eb7839ff7094d2f850L83
for test code