assert_select has been moved into the rails-dom-testing gem and is now
based on Nokogiri.
* Remove assert_tag and assert_xml_select
* Quote CSS attribute selector values
* Use decoded versions of HTML in assert_select. Nokogiri decodes
elements prior to matching.
* Add a test helper for entities such as ` ` for which it's
difficult or confusing to include the decoded version directly in the
assertion.
* Assert directly on Atom feeds' XML namespace because it isn't
selectable as an attribute
