Ensure that the character set is UTF-8 for RSS feed

When setting the Content-Type header for the RSS feeds, the character
set is omitted. However, when running the feed through the
[Feed Validator](http://feedvalidator.org), it became apparent that
there was a problem as the encoding defaults to US-ASCII. See
[RFC 3023](http://www.ietf.org/rfc/rfc3023.txt) for further information
on the default XML charset over HTTP.

I have added a test and simple update to the 'Content-Type' header.
Merge pull request #2642 from connrs/feature-add-charset-to-rss

Ensure that the character set is UTF-8 for RSS feed