CollectionInput will always use item_wrapper_tag

Fixes https://github.com/plataformatec/simple_form/issues/926

For radio or checkbox collection always use `:item_wrapper_tag` to wrap the content and add `label` when using `boolean_style` with `:nested`.

Output (using our [SimpleForm Bootstrap](https://github.com/rafaelfranca/simple_form-bootstrap) example):
```html
<div class="form-group check_boxes optional article_content_type"><label class="check_boxes optional control-<div class="form-group check_boxes optional article_content_type">
  <label class="check_boxes optional control-label">Stacked checkboxes</label>
  <div>
    <span class="checkbox">
      <label for="article_content_type_blog" id="article_content_type_blog" name="article[content_type]">
        <input class="check_boxes optional form-control" id="article_content_type_blog" name="article[content_type][]" type="checkbox" value="Blog">
        Blog
      </label>
    </span>
    <span class="checkbox">
      <label for="article_content_type_editorial" id="article_content_type_editorial" name="article[content_type]">
        <input class="check_boxes optional form-control" id="article_content_type_editorial" name="article[content_type][]" type="checkbox" value="Editorial">
        Editorial
      </label>
    </span>
    <span class="checkbox">
      <label for="article_content_type_announce" id="article_content_type_announce" name="article[content_type]">
        <input class="check_boxes optional form-control" id="article_content_type_announce" name="article[content_type][]" type="checkbox" value="Announce">
        Announce
      </label>
    </span>
    <span class="checkbox">
      <label for="article_content_type_advertisement" id="article_content_type_advertisement" name="article[content_type]">
        <input class="check_boxes optional form-control" id="article_content_type_advertisement" name="article[content_type][]" type="checkbox" value="Advertisement">
        Advertisement
      </label>
    </span>
    <input name="article[content_type][]" type="hidden" value="">
  </div>
</div>
```

Thanks for @kassio