Added ability to do XPath matching using XMLUnit.assertXpathExists

I came across a use case where I wanted to only check certain fields in a soap request, for example, I wasn't interested in the value of the modified date.  This allowed me to do the following in a test:

            verify(postRequestedFor(urlEqualTo("/someurl"))
                .withRequestBody(equalToXPath("""//*[local-name() = 'eventId'][.='update']"""))
                .withRequestBody(equalToXPath("""//*[local-name() = 'source'][.='Prioritisation']""")));