Configuration.setTransactionManagerLookup() is ignored

opposing to being able to set the classname only, it should be able to pass existing instances. I'm dealing with a case in which I have to create and configure a TransactionManager before starting the cache.

If it's simple enough, it would be very nice to have this in 4.2.1 too - feel free to remove the version if it doesn't fit.

Providing a testcase.