Add attribute for to label in collection extension

Fixes https://github.com/plataformatec/simple_form/issues/830

The motivation for this it's to add `for` attribute on `label` in `radio` collections to be valid in AA compliance (http://achecker.ca/checker/index.php).

Before:
```html
<div class="controls">
  <label class="radio">
    <input class="radio_buttons optional" id="article_content_type_blog" name="article[content_type]" type="radio" value="Blog">Blog
  </label>
  ...
</div>
```

After:
```html
<div class="controls">
  <label class="radio" for="article_content_type_blog">
    <input class="radio_buttons optional" id="article_content_type_blog" name="article[content_type]" type="radio" value="Blog">Blog
  </label>
  ...
</div>
```

Thanks for @ulissesalmeida and @fabioyamate for the pair programming