Backport:

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: https://issues.apache.org/bugzilla/show_bug.cgi?id=47106
+1: minfrin, jim, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@917010 13f79535-47bb-0310-9956-ffa450edef68