FIX: embedding topics would fail with some HTML

When truncating content we try to search for first paragraph, if HTML had
no P it would fallback to first div which may have nested elements.

app/models/topic_embed.rb

@@ -227,7 +227,7 @@ class TopicEmbed < ActiveRecord::Base
     return result unless result.blank?
 
     # If there is no first paragaph, return the first div (onebox)
-    doc.css('div').first
+    doc.css('div').first.to_s
   end
 
   def self.expanded_for(post)