#252 merging reverse mapping with forward mapping.

There is also an error in the `Mapper#reverse` method class.  The check on constant / expression has been fixed.

Also, the order of forward / reverse method is not relevant (as thought before). This is been assured by adding some tests.
That's an interesting case as there are two matching "forward" mapping methods. So which one is going to be used as configuration source? Is it determined in any way?