next callback should use the response payload

[tiswrt]

This PR was already made some time ago #34, but was rejected due to a missing test.

When using reply.etag() the etagOnSend hook uses the cache payload, instead of the response payload.

I have added a test that checks whether the correct payload is returned.

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

[jsumners]

Please follow the instructions in pinojs/pino-pretty#144 (comment) so what we can credit the original author's work.

[tiswrt]

I followed your instructions and created a new PR #49 . Closing this one