Correct mime-types for atom, rdf, and rss

This solves some issues with nosniff header also.

atom: application/atom+xml  (https://www.iana.org/assignments/media-types/media-types.xhtml)
rdf:    application/rdf+xml (http://www.w3.org/2008/01/rdf-media-types)
rss:   application/rss+xml (http://www.rssboard.org/rss-mime-type-application.txt)