Add some failing test when asserting body class
Envelop fragments in documents, and make document_root_element private again.

This reverts changes 02a40f7 and 6fc60c9.
Fix #26 by running assertion when we're aren't testing for a body.

We're creating Nokogiri documents from HTML fragments, which means Nokogiri automatically
adds a body element which is unexpected.
Merge pull request #27 from rails/special-body

Fix #26 by running assertion when we aren't testing for a body.