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 (using #scrub) which prevents the numerous
encoding errors.