Fixed duplicate textarea class attribute

[lcrea]

When a user sets the classes attribute for a textarea field, the Twig template duplicates the class attribute inside the textarea tag, resulting in a not valid HTML like this:

Markdown source

- name: message
  classes: form-control
  type: textarea

Final HTML

<div class="form-textarea-wrapper">
    <textarea class="form-control" name="data[message]" class="form-control"></textarea>
</div>

[flaviocopes]

👍