Fixes for sending messages including multibyte characters

I tried to post messages including multibyte characters (Japanese) but I couldn't. And I received error messages below. I solved this problem by this change.

```
2013-11-16T14:49:05.288283+00:00 app[web.1]: Sending message
2013-11-16T14:49:05.335856+00:00 app[web.1]: No payload received
2013-11-16T14:49:05.335856+00:00 app[web.1]: Slack services error: 500
```