Merge r1483027 from trunk:

mod_proxy_http: Make the proxy-interim-response environment variable
effective by formally overriding origin server behaviour.

Submitted by: minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1502198 13f79535-47bb-0310-9956-ffa450edef68