Correctly align the behavior of headers_in to be consistent with the

treatment of headers_out, resolving PR 48359 by keeping subrequest
scope changes out of the main request headers.  This ensures that all
requests-without-bodies behave as the requests-with-bodies code has.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@901578 13f79535-47bb-0310-9956-ffa450edef68