Support virtual columns in union queries

[dgollahon]

Describe the bug

Queries with virtual columns will drop or overwrite information when used as part of a union query. This is a regression as a result of #334.

To Reproduce

I opened a failing test PR here which also has a better explanation of some of this behavior.

Expected behavior

Virtual columns can be used in union queries, like before #334.

My environment

• Affects my production application: Yes
• Ruby version: 2.7
• OS: MacOS

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I opened the corresponding PR a couple months back hoping a failing test would be a helpful way to surface this. Trying an issue in case that's a better pathway. Also cc @ianks (since you authored #334) and @flash-gordon since you reviewed the change in case you have any additional context/advice.

Context: I am trying to upgrade my rom-sql version from 2.x to 3.x and I have code that relies on this behavior that is hard to work around. I can monkey patch what is effectively a revert of #334 but was hoping for an upstream fix and/or maybe some context/advice. I am not sure I have a good intuition for what a proper fix would be but I could try to take a stab if someone will have bandwidth to review.

Thanks!