composite column on joined subclass doesn't get loaded on >= r6199

A composite column on a joined table subclass doesn't get populated (it is None) when the object is loaded. It was introduced in r6199, which solves a similar problem with ColumnProperty's but creates this problem.

A test case is attached, expressed as a diff against the current trunk (r6498). Picking out just the r6199 commit and rolling it back makes the test pass, but causes the ColumnProperty tests to break.