Mark translations of _html suffixed keys as html safe.

It used to escape translations of keys suffixed with `_html`, which made it inconsistent with Rails behavior. In Rails when we have a key like `some_key_html`, it's translation is marked as html safe, as we can see in https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/translation_helper.rb#L46

This fix allows the use of `_html` suffixed keys for collection options.

Previous to this, if someone wanted to workaround this limitation, it was necessary to provide a `label_method` such 

`->(item) { t(item[1]) }`

which calls the `translate` method referred above.

I'm not completely sure if it should be done in other places or if this is the best approach.