force s3 backup on post replacements if md5 doesn't change (ref #3290)

app/models/post_replacement.rb

@@ -57,6 +57,8 @@ class PostReplacement < ApplicationRecord
       if md5_changed
         post.comments.create!({creator: User.system, body: comment_replacement_message, do_not_bump_post: true}, without_protection: true)
         ModAction.log(modaction_replacement_message)
+      else
+        post.queue_backup
       end
 
       post.save!