Merge r1640495, r1644031 from trunk

   * mod_proxy_fcgi: Ignore body data from backend for 304 responses. PR 57198.

Submitted by: jkaluza
Reviewed by: jkaluza, ylavic, covener
Backported by: jailletc36

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1650677 13f79535-47bb-0310-9956-ffa450edef68