fix inferring the correct fn type

[Uzlopak]

Fixes #217

I got help from @webstrand. Props to him.

It should now work.

According to webstrand

Typescript isn't currently capable of partial inference
if you specify any of the type parameters, the rest get filled in with their defaults
this is why you see some typescript-specific apis doing stuff like
foo()("blah") because the first function call doesn't perform any inference, but typescript lets the second function call infer from its parameters

Maybe needs a new paragraph regarding typing in the readme.md were we basically write, that they should not use the generics but specify the types on the parameters, when calling fastifyPlugin?

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[Uzlopak]

@fastify/typescript

Can you have a look please?

I am actually highly confident, that this is the solution.

[fox1t]

Hey. This approach is new to me: instead of using polymorphism, we are going with a generic inference. I like it! I am wondering if there are other points in the Fastify ecosystem where we can use the same approach (the same one we did with the infamous triplet and the export = thing when we discovered it)

[mcollina]

How? Could you open a fresh issue?

[Uzlopak]

@mcollina Probably mixed use of async and callback. like we had in #220
Should we add the infomation, that you should not mix both approaches?

[mcollina]

Should we add the infomation, that you should not mix both approaches?

yes

[Uzlopak]

created an issue #221