Column._copy() duplicates "user defined" nullable state exactly

Column._copy() duplicates "user defined" nullable state exactly

To accommodate how mapped_column() works, after many
attempts to get this working it became clear that _copy()
should just transfer "nullable" state exactly as it was,
including the state where .nullable was set but user_defined_nullable
remains at not user set.

additionally, added a similar step to _merge() that was needed
to preserve the nullability behavior when Identity is present.

server / client default objects are not copied within column._copy()
and this should be fixed.

Fixes: #8410
Change-Id: Ib09df52b71f3e58e67e9f19b893d40a6cc4eec5c