Switched FunctionWriteFile content parameter to xalan::XalanNode pointer
* i.e. "write-file" now supports the serialization of given DOM structures
** this will be needed e.g. if FunctionTransform returns the result as a DOM tree instead of as a plain string
** enables the creation of multiple XML documents from within a single transformation
*** i.e. backports the functionality provided in XSLT 2.0 by "xsl:result-document" to xalan's XSLT 1.0
* changed test cases accordingly