Add controller test, assert inclusion of RSS <link>

Per @steveklabnik’s [suggestion][c] to add the previously in-line [pull
request test][prt] to the project.

[c]: https://github.com/artisan-tattoo/artisan_assistant/pull/10#issuecomment-46135726
[prt]: https://gist.github.com/andrewsardone/66f8dacafb0e63feb065