force heading slug change (#2122)

app/models/communication/block/heading.rb

@@ -93,6 +93,11 @@ class Communication::Block::Heading < ApplicationRecord
 
   protected
 
+  def set_slug
+    self.slug = nil
+    super
+  end
+
   def slug_unavailable?(slug)
     self.class.unscoped
               .where(slug: slug, about_type: about_type, about_id: about_id)