[Bug] Relation#union fails on virtual columns

[dgollahon]

• Merging relations with virtual columns the qualified projection (added in #334) causes the virtual column to be moved into the outer select. This means that column will always be hardcoded to NULL instead of selecting from the result of the union. This is a not-that-esoteric technique with unions. This also worked in ROM prior to #334.

This PR is really an issue/bug report just in the form of a failing test. I don't have a good idea as to what the right solutin is but I figured it would be helpful to at least have a very concrete demonstration of the issue.