Sanitize value in label for

The id of the radio is sanitized but the `for` attribute of the label was not.
I copied the sanitize method from Rails: https://github.com/rails/rails/blob/f04dd33f330c96cdcf0b7a3728a9c2502db6a4c2/actionview/lib/action_view/helpers/tags/base.rb#L114-L116