cleanup log_header_out function:

- no need to ask r->err_headers_out, because it's already merged with
  r->headers_out at this stage
- allow multiple headers like Set-Cookie to be logged properly

PR: 27787


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@103331 13f79535-47bb-0310-9956-ffa450edef68