fix text view to use tinymce serializer

### Issue
When using a tinyMCE plugin that relies on a `parser`/`serializer` `NodeFilter` it would not work properly in a editable elements text view.

### Reason
`.getBody().innerHTML` is used instead of the proper tinyMCE API [`.getContent()`](http://www.tinymce.com/wiki.php/API3:method.tinymce.Editor.getContent) which would trigger the appropriate callbacks.

#### Side effects of `.getContent()`
Using `.getContent()` will encode umlaut and other utf8 things to HTML entities. Which already happens in `content_entries/form_view` (calls `.save()` which calls `.getContent()`) and is most likely the desired behaviour.

### Test
It would be relatively simple to test in a JS unit spec. My understanding is you do not have any JS unit specs? A cucumber feature seams wrong for this - up to you to decide if its worth testing.