Changes RSS item creation to prevent encoding errors

SimpleRss is unreliable with parsing RSS feeds that contain German Umlauts. 
For example this feed http://www.lauffeuer-lb.de/api/v2/articles.xml can't be parsed by SimpleRss. Discourse's logs are full of  

```
Job exception: Wrapped Encoding::CompatibilityError: incompatible character encodings: ASCII-8BIT and UTF-8  
Job exception: incompatible character encodings: ASCII-8BIT and UTF-8
```

The embedding fails because the feed can't be parsed.  

This change forces the encoding which prevents the numerous encoding errors.

app/jobs/scheduled/poll_feed.rb

@@ -86,11 +86,15 @@ def url
       end
 
       def content
-        @article_rss_item.content || @article_rss_item.description
+        if @article_rss_item.content
+          @article_rss_item.content.force_encoding(Encoding::UTF_8)
+        else
+          @article_rss_item.description.force_encoding(Encoding::UTF_8)
+        end
       end
 
       def title
-        @article_rss_item.title
+        @article_rss_item.title.force_encoding(Encoding::UTF_8)
       end
 
       def user