HeuristicMixedException reported to client, although backends don't support heuristic completion

this bug is related to:
https://issues.jboss.org/browse/JBTM-1865

While testing recovery and transaction failures we came across
two related issues, the test scenario is almost the same, the other
issue is filed as JBTM-1865

Scenario:
2 enlisted XAResources (one for each database used by the test),
The prepare phase is successful.
We crash the second database server during the XAResource.commit invocation,
after it has committed it's branch, right before the control flow would return to the XAResource (which as a result throws a XAException.XA_RBCOMMFAIL).
The TM can not know at this point in time, whether the database successfully committed the transaction or not. In this particular case, all changes have been committed.
By definition of the 2PC protocol, the transaction should be considered as "to be committed", because the prepare phase has ended successfully.
The first branch was successfully committed (as it should be spec.)
But now things go awry:
1. The client receives a HeuristicMixedException, although the backend
does not support heuristic completion at all, i.e. there should be no Heuristic* exception be thrown

The correct behavior is, to report the transaction as success to the client, commit all other branches. All XAResources which failed during XAResource.commit() are supposed to be recovered in future.