Add a failing test: XML templates are rendered as HTML5

This is more of a bug report than a pull request but I may continue to add commits to the pull request as time permits.

For now this pull request includes a new test with an XML file which has a link tag that highlights a bug in Heist where a `*.xtpl` template is loaded/rendered as HTML.

Just having the `rss.xtpl` in the templates directory causes most tests to fail which leads me to believe that the file is getting loaded as HTML and not XML.

At this point my plan is to try to fix the bug this weekend.