Merge r1775858 from trunk:

Return HTTP 504 rather than 503 when a proxy timeout is hit.

Part of the review for PR: 56188


Submitted by: elukey
Reviewed by: elukey, jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1783414 13f79535-47bb-0310-9956-ffa450edef68