#278 fix for applying expressions in the reverse, test for Boolean isPro...

...perty method workaround

Fixing problem in 'org.mapstruct.ap.model.source.Mapping'. 

Adding a unit test for demonstrating how to get around the target accessor isProperty for Boolean target accessors by means of expressions, which we can use to refer to when the  -enableIntrospection switch to XJC (JAXB) is not an option.

The unit test also demonstrates the fix (of course).