ActiveAdmin filter boolean as single checkbox
===

How can i filter a boolean value using a single checkbox?

For example the following gives me two checkboxes with `yes` and `no` values:

    ActiveAdmin.register User do    
      filter :is_retired, :as => :check_boxes
    end

HBTM association is filtered as follows:

    filter :roles_id, :as => :check_boxes, :collection => proc { Role.all }

But i have a boolean field in my `User` model and need a single checkbox with `Is retired` label. Is it possible somehow?