mod_proxy_http: Make sure that when an ErrorDocument is served

from a reverse proxied URL, that the subrequest respects the status
of the original request. This brings the behaviour of proxy_handler
in line with default_handler. PR 47106.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@909899 13f79535-47bb-0310-9956-ffa450edef68