Add test to demonstrate validation issue

[dub-wt]

[grape-bot]

	1 Warning
:warning:	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

	1 Message
:book:	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#2474](https://github.com/ruby-grape/grape/pull/2474): Add test to demonstrate validation issue - [@dub-wt](https://github.com/dub-wt).

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