fix: error from payload

[nickchen]

Proposed changes

fix the error from out of order Close for Writer. This cause the following error on 1.19:

flate: closed writer

changed the hash in the test, but added new validation against file contents.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

[sanathkumarbs]

LGTM, is the change backward compatible if someone is running go 1.18?

I see that the go.mod pins to 1.18:
https://github.com/nginx/agent/blob/main/go.mod#L3

[nickchen]

LGTM, is the change backward compatible if someone is running go 1.18?

I see that the go.mod pins to 1.18: https://github.com/nginx/agent/blob/main/go.mod#L3

should be, the agent is still on 1.18, and the pipeline tests passed.