Sorry for the late reply. WSTrustClient uses the JAXWS dispatch call. I am guessing that is what makes the content type to be text/xml.
the WCF endpoint is probably looking for SOAP based clients.
Try with SOAPConnection from the SAAJ API.
MessageFactory messageFactory = MessageFactory.newInstance();
SOAPMessage soapMessage = messageFactory.createMessage();
//Create the SOAPMessage somehow
SOAPConnectionFactory connectFactory = SOAPConnectionFactory.newInstance();
SOAPConnection connection = connectFactory.createConnection();
//Send it across the wire
URL url = new URL(endpoint);
SOAPMessage response = connection.call(soapMessage, url);
I am neither a fan nor an expert of Web Services.