Merge r1588519 from trunk:

mod_proxy: When ping/pong is configured for a worker, don't send or forward
           "100 Continue" (interim) response to the client if it does not
           expect one.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1608762 13f79535-47bb-0310-9956-ffa450edef68