fix: don't modify options

[SkeLLLa]

Better options handling. Usually most configuration objects are frozen, so it adds slightly better experience for using it with other modules and better for debugging.

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 Code of conduct

[mcollina]

lgtm, do you need this backported to the fastify v2 line as well?

[SkeLLLa]

@mcollina we can, but spread was introduced without flags in node 8.3.0 (as far as I remember). And for fastify v2 there were tests for node 6.

Basically I've added this, because old node versions were removed from tests.

[jsumners]

A back port can be accomplished with Object.assign. It is effectively equivalent in this case.