Fix casing in type_of_union for union types

[hdobbelaere]

Before this PR

After this PR

==COMMIT_MSG== ==COMMIT_MSG==

Possible downsides?

[palantirtech]

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[changelog-app[bot]]

Generate changelog in changelog-dir>`changelog/@unreleased`</changelog-dir

Type

- [ ] Feature - [ ] Improvement - [x] Fix - [ ] Break - [ ] Deprecation - [ ] Manual task - [ ] Migration

Description

Fix casing in type_of_union for union types **Check the box to generate changelog(s)** - [x] Generate changelog entry

[svc-autorelease]

Released 4.7.0