#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.
> You remarked this before. 

Ah, I see. No need to change it in this PR, it's more a general comment.

> also UT should be a bit fun to read :smile:

+1 :-)

Not sure about using shared models for tests (if that's what you meant by "repository"), as it may make tests harder to comprehend and create dependencies between them. Better have separate test models per test case.