Capture output from XML::Writer

 XML::Writer::String implements a bare-bones class specifically for the
 purpose of capturing data from the XML::Writer module. XML::Writer expects an
 IO::Handle object and writes XML data to the specified object (or STDOUT) via
 it's print() method. This module simulates such an object for the specific
 purpose of providing the required print() method.
 .
 It is recommended that $writer->end() is called prior to calling $s->value()
 to check for well-formedness.