Fix the merge of a label class defined inside the block

In the `collection_check_boxes` and `collection_radio_buttons`, if we try to define a class inside a block to a `label` tag, it won't be applied.

For example:

```erb
<%= form_for @form do |f| %>
  <% collection = [[1, true, { class: 'foo' }]] %>
  <%= f.collection_check_boxes :options, collection, :second, :first do |b| %>
    <%= b.label(class: 'my_custom_class') %>
  <% end %>
<% end %>
```
the class `my_custom_class` won't be applied to the label.

Also, following the @rafaelfranca suggestion I created a whitelist of which input attributes modifies the label `for` attribute.