Union doesn't honor unicode types

Given a CompoundSelect (in my real case, a union()) which involves Selects with unicode columns, when fetching its ResultProxy we get RowProxy'ies that doesn't honor the original string column type (unicode columns become normal string columns).

This problem doesn't occur with sqlite (at least in memory), but it does with Firebird and Postgres (didn't test other databases).

This bug happens with both 0.4.8 and 0.5rc2 SA versions.