TypeError: Object true has no method 'replace'

Hi, thanks for this library.
Trying the example, when I run `mosquitto_pub` I get:

```
/usr/local/lib/node_modules/mqtt-exec/mqtt-exec.js:59
var message = message.replace(/"/g, "\\\"");   
                      ^

TypeError: Object true has no method 'replace'
```

It looks like the message "true" becomes a buffer object and not a string. (`<Buffer 74 72 75 65>`)
