#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).  
Why is this mapping required here? Shouldn't this not be automatically be done? IMO, the other method doesn't qualify as configuration source (by means of reversing), as it has a an expression and I feel we should exclude those when looking for reverse methods (as the expression cannot be reversed). Or am I missing something?