Many thanks for the thorough tests!

Regarding the usage of `@WithClasses`, it seems as if many of the referenced types are the same for all/several test methods. I'm wondering whether we should merge the type lists given on the class and the method level. That way one could specify a basic list with classes used in all test methods on the class level and only add the specific types on the method level.

WDYT? That'd require a small fix in `MapperTestBase#getTestClasses()`.