- The ``__mapper_args__`` dictionary is copied from a declarative

- The ``__mapper_args__`` dictionary is copied from a declarative
mixin or abstract class when accessed, so that modifications made
to this dictionary by declarative itself won't conflict with that
of other mappings.  The dictionary is modified regarding the
``version_id_col`` and ``polymorphic_on`` arguments, replacing the
column within with the one that is officially mapped to the local
class/table. fixes #3062