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.
Feature to allow each imported post to be created using a different discourse
username. A possible use case of this is a multi-author blog where discourse
is being used to track comments. This feature allows authors to receive
updates when someone leaves a comment on one of their articles because each of
the imported posts can be created using the discourse username of the author.
