Default implementation of Effigy creates invalid HTML5 markup

[jmreidy]

Effigy views create their output using the "output" method, which defaults to using Nokgiri's "to_xhtml" method. This method appends "xmlns" info to the root tag, regardless of whether the HTML document actually has an XHTML doctype. This issue can be resolved by overiding "output" and calling current_context.to_html instead.

It might be helpful for docs to be updated to reflect this use case.

[jferris]

Output valid HTML by default instead of XHTML; closed by eedd38b