Fixed decorator for multiple roots

[Jelenkee]

Just a fix for a bug I found yesterday.

Checklist

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

[mcollina]

lgtm

[climba03003]

Doesn't https://github.com/fastify/fastify-static/blob/master/index.js#L46 already merged sendOptions.root ?
Why we need to pass one more times on https://github.com/fastify/fastify-static/blob/master/index.js#L165 ?

[Jelenkee]

@climba03003 It is merged. But the root may be an array. And the send stream cannot handle an array as root. That's why it is transformed here