fix: content encoding priorities

[shishir-karanth]

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

[climba03003]

Can you add an unit test to ensure it works properly?

You can checkout the existing test inside https://github.com/fastify/fastify-compress/tree/master/test as example.