[#1504] Atom Feed HTTP Content-Type header is set to text/html.

With this change, Content-Type is set to 'application/atom+xml'
Merge branch '1504-atom-feed-content-type' of https://github.com/george-sattler/ckan into george-sattler-1504-atom-feed-content-type
Merge branch 'george-sattler-1504-atom-feed-content-type'