Fix SAXSource handling in AbstractJaxb2PayloadMethodProcessor

The current code basically assumes that if the payload is a SAXSource,
then it must be backed by a SAX parser. It then passes only the
InputSource to the unmarshaller and ignores the XMLReader. Presumably
this is an optimization because the unmarshaller will likely parse the
XML message in a more efficient way.

This approach causes a problem if the SAXSource is not backed by a
parser but generates the sequence of SAX events using a custom
XMLReader.