- A column can be added to a joined-table subclass after

- A column can be added to a joined-table subclass after
the class has been constructed (i.e. via class-level
attribute assignment).  The column is added to the underlying
Table as always, but now the mapper will rebuild its
"join" to include the new column, instead of raising
an error about "no such column, use column_property()
instead".  [ticket:1523]
- added an additional test in test_mappers for "added nonexistent column",
even though this test is already in test_query its more appropriate within
"mapper configuration" tests.