Revert "FIX: Use #dup instead of #+@ since content could be an instance of Nokogiri::XML::Element."

This reverts commit 50afe59306.

app/models/topic_embed.rb

@@ -37,7 +37,7 @@ class TopicEmbed < ActiveRecord::Base
       contents = first_paragraph_from(contents)
     end
     contents ||= ''
-    contents = contents.dup << imported_from_html(url)
+    contents = +contents << imported_from_html(url)
 
     url = normalize_url(url)