Wrong SOAPContentElement namespace prefix after multiple model transitions

This was spotted because of JBPAPP-3785. Basically the problem is when moving from XML_VALID to DOM_VALID state of the model, the Native stack can end up with a wrong namespace assigned to the SOAPContentElement of the body. That's possible when a previous transition from OBJ_VALID to XML_VALID has caused JAXB to marshall the content to a new xml fragment that despite being equivalent to the original content has different namespace <--> prefix associations. When later moving to DOM_VALID, attributes are copied from the element obtained parsing the fragment to the container element. This breaks messages in some subtle cases because the SOAPContentElement prefix is not updated.