Ensure each subrequest has a shallow copy of headers_in so that the

parent request headers are not corrupted.  Eliminates a problematic
optimization in the case of no request body.  

PR: 48359 
Submitted by: Jake Scott, wrowe, rpluem
Backports: server/protocol.c r901578
Reviewed by: minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@917867 13f79535-47bb-0310-9956-ffa450edef68