MULE-6501 - Handle XSLT parameters with expressions that return (XML) objects

MULE-6501 - Handle XSLT parameters with expressions that return (possibly XML node) objects.

Key use-case now supported is extracting a fragment of an XML payload
via an #[xpath()] expression, and later using this as a input parameter
to an XSLTTransformer.

1. XsltTransformer calls ExpressionManager.evaluate() instead of
ExpressionManager.parse()
2. Converts the returned object to a javax.xml.transform.Source if it is
convertible.
  * XMLUtils.toXMLSource() handles conversion of Dom4j nodes as well
as complete documents.
3. Otherwise uses the expression result object as-is for the XSLT
parameter.

The expanded support for Dom4j nodes is because the xpath() function
might return a w3c DOM or Dom4j node depending on what format the
message payload was in. Converting the XSLT parameter to
java.xml.transform.Source allows Saxon to process an XML node parameter
as intended.
use stringValue as per mule style guide