Don't use ActionView::Helpers::InstanceTag.check_box_checked? anymore

Even though ActionView::Helpers::InstanceTag exists in Rails 4, the check_box_checked? method is gone, so the defined? check that was put in earlier did not have the desired effect.

I propose to always use our own boolean_checked? method since that works across all Rails versions.

This fixes the rendering of check boxes in Rails 4 (edge Rails).