Infinite $digest Loop when using $sce with filter

This example http://jsfiddle.net/FmeP9/ produce Error: [$rootScope:infdig] using this code:

```
filter('markdown', function ($sce) {
    var converter = new Showdown.converter();
    return function (value) {
        var html = converter.makeHtml(value || '');
        return $sce.trustAsHtml(html);
    };
});
```

It is working but i think perfomnce will go down if those error shows up.
