Problem: when replacing the root element, UpdateXML
erroneously tried to mix old XML content with the
replacement string, which led to crash.
Fix: don't use the old XML content in these cases,
just return the replacement string.


mysql-test/r/xml.result:
  Adding test case
mysql-test/t/xml.test:
  Adding test case
sql/item_xmlfunc.cc:
  Adding special code to handle replacements of the root element -
  the replacing content is just copied to the result,
  the previous content of the XML value is removed.