#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).  
Yes. 
@bbaetz actually provided this example via a gist. I think after a remark from you some while ago that expressions could be used to work around the problem. See @278.  I just reworked it as a UT to demonstrate it works. 

To solve the "is" -prefixed method, we probably should include some kind of configuration that considers the is prefix in combination with a wrapped type Boolean.