Fix for missing handling of CDATA elements in the XmlMini LibXML engine and more REXML-like whitespace handling.

Hi there,

I have submitted lighthouse ticket #3313 which includes a fix for
missing CDATA handling in the XmlMini libxml engine. Without the fix,
the engine ignores content inside CDATA blocks and ends up producing
nil Hash keys.

The patch attached to the ticket also provides a test for the libxml-
engine and adjusts its whitespace handling to bring it more in line
with that of the default REXML engine.