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)

spec/models/topic_embed_spec.rb

@@ -105,6 +105,16 @@ describe TopicEmbed do
 
         expect(post.raw).to include(long_content)
       end
+
+      it 'looks at first div when there is no paragraph' do
+
+        no_para = "<div><h>testing it</h></div>"
+
+        SiteSetting.embed_truncate = true
+        post = TopicEmbed.import(user, url, title, no_para)
+
+        expect(post.raw).to include("testing it")
+      end
     end
   end