Update assert_select for Nokogiri compatibility

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

test/support/html_entity_helper.rb

@@ -0,0 +1,11 @@
+require 'htmlentities'
+
+module HTMLEntityHelper
+  def entity(name)
+    HTMLEntities.new.decode("&#{name};")
+  end
+
+  def nbsp
+    entity('nbsp')
+  end
+end