Fix ProxyAddHeaders merging.

* modules/proxy/mod_proxy.h:
  Add add_forwarded_headers_set field to proxy_dir_conf.

* modules/proxy/mod_proxy.c (create_proxy_dir_config):
  Initialize add_forwarded_headers_set.
  (add_proxy_http_headers): Set it.
  (merge_proxy_dir_config): Merge add_forwarded_headers correctly.

Reviewed by: jorton, rpluem, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1805390 13f79535-47bb-0310-9956-ffa450edef68