Nokogiri XmlMini backend doesn't properly parse cdata elements

In a production project I have I'm using Hash#from_xml to parse a bunch of XML from a foreign CMS. I found that changing the backend to Nokogiri gives me an over 100% speed boost (woo!), but the backend unfortunately doesn't process CDATA sections correctly.

This small patch allows the nokogiri backend to parse CDATA sections and adds a test. It should apply to both 2-3-stable and master.

There's also a pending patch to clean-up this nokogiri backend from Aaron Patterson that is pending -- #2243 Nokogiri back end should not be so clever