Make sure JBXB implements support for unmarshal(Source)

The issue here is that WS has to use org.jboss.wsf.common.DOMWriter to convert (DOM)Source to a String to be able to pass it to JBXB. There is already an issue (JBXB-100) that requests a support for different interface. I reopened the issue and tried to explain why there is such a need for the support.

The thing is that DOMWriter.normalize() method calls (that are really inefficient and cause performance issue, see JBWS-2853) wouldn't be necessary then.