Handle access_log_headers in proxy_logging

 * in etc/proxy-server.conf-sample, there is an access_log_headers directive in proxy_logging middleware.
   but current code doesn't handle it. This patch enables proxy_logging to handle access_log_headers.

Change-Id: I03337ccc4f48625af3f578ed543da992d3eec2bd
Merge "Handle access_log_headers in proxy_logging"