jtax subordinate transactions throw unnecessary heuristics

The SubordinateTestCase is showing up cases where the JTAX version of subordinate transactions is throwing HeuristicCommit or HeuristicRollback in cases where this should not be necessary. For example

begin()
setRollbackOnly()
commit()

throws RollbackExcpetion in the JTA version but a heuristic in the JTAX flavour. It would be better if the behaviour was consistent between the two.