Keeps dirty form class in body if output is silenced.

Hi!

I was using this plugin with the `silence: true` setting, and I needed to verify if a form was dirty before submitting.

I noticed the `dirty` class seemed to be disappearing, so I fixed it. Not sure if there was a reason for that.

Here's my code:
```javascript
$('form').on("submit", function(){
  if($(this).hasClass('dirty')) { 
    // do something 
  }
});
```

Thanks for the plugin :beers: 

Bruno