[fix] ensure serialized headers check is always applied

[dummdidumm]

Closes #7112 - if we decide this is enough, and that we don't want some minimum whitelist. At least content-type seems like an easy win, and would save people some hassle - for example from #6412 (comment) it seems that some libraries want the content type header.

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[dummdidumm]

I'm wondering why the proxy is used instead of monkey patching the other methods as well - convenience?

Also: Should we apply this check in DEV only?

[Rich-Harris]

It would be more convenient to monkey-patch the Response methods instead of using the proxy, but we get 'illegal invocation' errors. For whatever reason that doesn't apply to Headers so we may as well do the convenient thing there.

I think it's fine to keep the error in production, since it doesn't result in extra code shipped to the client and might help someone diagnose a bug