Enable rep_header ACL in reply_header_access

[chi-mf]

If a response-dependent ACL is used in reply_header_access, Squid warns: "ACL is used in context without an HTTP response. Assuming mismatch".

[squid-prbot]

Can one of the admins verify this patch?

[yadij]

OK to test

[yadij]

Updated the title and description to fit within commit requirements.

[rousskov]

I added an S-waiting-for-more-authors label to mark useful PRs that are waiting for development work that the original author is unable or unwilling to do. Such PRs should probably be closed as abandoned after some timeout.