- add additional logic that duplicates mapper's prop.copy(); prop.columns.append...

- add additional logic that duplicates mapper's prop.copy(); prop.columns.append(col)
logic when columns are present in a joined subclass with an attribute name different
than the column name itself [ticket:1931]
- add coverage to verify that we need to check (obj.name or name) when
deciding if a Column from a mixin should be added to the mapped table