Force text/xml as MIME-type in the feed

In the last versions of WordPress the feed can be generated as application/rss+xml, application/rss+xml, text/xml, application/atom+xml' or application/rdf+xml

Some of these content-types are not recognized by old feed readers or some JAVA feed managers (as Jsoup) generating a validation exception.

This plugin overwrite the content-type by 'text/xml' used in the past.