Fixed what appears to be a bug in MappingProcessor. It appeared that the
mapId for objects within collections was not being passed to the
call to mapToDestObject which results in incorrect mapping. For example
when specifying TypeMappingOptions.mapNull(false) nulls were still mapped.

Have added a unit test DeepMappingWithMapIdTest which fails with the unpatched code and passes with
the patch applied.
Merge pull request #136 from martin-d-ball/master

Fixed what appears to be a bug in MappingProcessor.