mod_proxy_fcgi: Fix sending of response without some HTTP headers

that might be set by filters.

The problem occurs when no body bytes were read while reading the
response headers, resulting in an empty brigade being sent down
the filter stack.  One particualr filter that mishandles the empty
initial brigade is mod_deflate.  It neglects to add to the response
header fields.

PR: 55558
Submitted by: Jim Riggs <jim riggs.me>
Reviewed by: trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1585824 13f79535-47bb-0310-9956-ffa450edef68