Validate body argument of return handler

[kornicameister]

There was first attempt to implement that validation in #160 but it proven to mypy quite cumbersome to try and implement that. It might be possible to get back to that with mypy plugin.

List of possible things to check for body:

• typing.Any with warning
• OAS has single response and its code is 204 -> there should be no body returned from handler
• OAS has multiple response and there are bodies there
• OAS defines multiple mimetypes for each code (hardest)

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