Test XML Documents Using assert_select

Have you ever wanted to test your API output without those pesky
assert_select error messages? Well now you can!

http://dev.rubyonrails.org/ticket/8173

The patch has been sitting there quietly with no comment for a couple
months now. I think it should be pretty easy to apply. It won't
affect standard HTML output or even XHTML given IE's inability
understand the standard application/xhtml+xml, and even if HTML was
served as HTML it would only raise warnings on invalid markup.

Thoughts?