Event handlers of sagas under test are not called if a GenericEventMessages is used as parameter to whenPubishinA

The event handlers of sagas under test by `AnnotatedSagaTestFixture` are not called properly if the event is wrapped in a `GenericEventMessage` or other subclasses of `EventMessage`. However, this is sometimes necessary as code in sagas might depend on meta data etc. which can only be passed by wrapping the event in tests.

Related issue: http://issues.axonframework.org/youtrack/issue/AXON-285