The XmlMini LibXML engine doesn't have a test, does not support CDATA elements, and treats whitespace differently from the REXML engine

The XmlMini LibXML engine cannot handle documents with CDATA blocks and treats whitespace differently than the REXML engine. The engine ignores content inside CDATA blocks and end up producing nil Hash keys.

I have written a test for the LibXML engine (it didn't have one) that exposes both of these issues along with a small patch which modifies activesupport/lib/activesupport/xml_mini/libxml.rb so that the test passes. The result is a version of libxml.rb which (a) handles CDATA elements and (b) treats whitespace similarly to the REXML engine. The test was based on the existing Nokogiri test.