Merge r1588527 from trunk:

mod_proxy: Preserve original request headers even if they differ
           from the ones to be forwarded to the backend. PR 45387.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1651082 13f79535-47bb-0310-9956-ffa450edef68