Fix support for latest Joi

[mcollina]

I have not added any unit test because we cannot really test latest joi as it requires Node 8+. It works on my machine.

cc @cemremengu

Fixes #1158.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• commit message and code follows Code of conduct

[delvedor]

Is this the only way to support it?
I would prefer a general purpose tool, such as the schema compiler instead have a custom specific code just to support an external library.

[mcollina]

I can check if it is not a literal (e.g it has a constructor headers.constructor !== Object) but I think this is cleaner and we can extend that if there si a need. The reality is that we shouldn't be implying json schema in that specific function.

[delvedor]

I think it would be better to figure out a general purpose approach.
Add this kind of code binds us to a specific library and change something in the future could lead to massive breaking changes. As far as I remember this is not the first time that Joi causes some issue...

[mcollina]

@delvedor PTAL this is generic.

[cemremengu]

I guess a more general purpose approach than this would be extracting validation into a plugin and just provide the interface inside the core? Then we could have:

fastify-validation-ajv
fastify-validation-joi

Just some random thoughts

[delvedor]

LGTM

[github-actions]

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