Champ: export multiple_drop_down_list as a [].join(', ')

app/models/champ.rb

@@ -77,6 +77,8 @@ class Champ < ActiveRecord::Base
         ActionView::Base.full_sanitizer.sanitize(value)
       when 'yes_no'
         value == 'yes' ? 'oui' : 'non'
+      when 'multiple_drop_down_list'
+        drop_down_list.selected_options_without_decorator(self).join(', ')
       else
         value
       end