Merge r327008 from trunk:

keep the proxied Content-Length header for a HEAD response. 

PR: 18757
Submitted by: gregames
Reviewed by: jorton, jerenkrantz, niq, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@354800 13f79535-47bb-0310-9956-ffa450edef68