Major Description
Ran into a sample that had an extra creator method in the ObjectFactory that returned a JAXBElement. When this method exists, and the jaxbcontext is created using the String signature in a user's JAXB context resolver, the unmarshaller will call the ObjectFactory method that returns the JAXBElement, and thus the provider will return a JAXBElement, which may not be the desired object for the resource – this would result in an IllegalArgumentException: argument type mismatch when calling the resource method.
See patch, with test. It'll make sense.
Also, we don't have a good set of tests for the JAXBXmlProvider. The one included here in the patch is a good one to use for additional JAXB tests.