#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).  
Hmmm. I just spent an hour tracing to figure out I was looking at a problem that I already fixed in PR #266

Around line 456 in `MapperCreationProcessor`, : `if ( method.getMappings().containsKey( sourcePropertyName ) )`. This method also triggers on an empty array which was just inserted at line 419.

I'll check in the fixes.