more unit tests for coercion

[dnesteryuk]

What do you think about unit tests for such cases? 🙂

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

	1 Message
📖	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:

* [#1978](https://github.com/ruby-grape/grape/pull/1978): More unit tests for coercion - [@dnesteryuk](https://github.com/dnesteryuk).

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[dblock]

I like me tests.