Add #for_api methods to CheckboxChamp and YesNoChamp

Otherwise, we return nil instead of “off” / “non”,
so we don’t expose the data properly

app/models/champs/checkbox_champ.rb

@@ -12,4 +12,8 @@ class Champs::CheckboxChamp < Champ
   def for_export
     value == 'on' ? 'on' : 'off'
   end
+
+  def for_api
+    value == 'on' ? 'on' : 'off'
+  end
 end

app/models/champs/yes_no_champ.rb

@@ -13,6 +13,10 @@ class Champs::YesNoChamp < Champs::CheckboxChamp
     processed_value
   end
 
+  def for_api
+    processed_value
+  end
+
   private
 
   def processed_value