Don't override cache-control if it has already been set

[prevostc]

Resolves https://github.com/fastify/fastify-caching/issues/14

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [?] documentation is changed or added -> not sure how to document this.
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct