Merge r1754164, r1801994, r1801995 from trunk:

mod_proxy_wstunnel: we want to detect whether some response was sent to
the client when forwarding data from the backend to the client, not the
reverse.


Follow up to r1754164: CHANGES entry.

Follow up to r1801994: CHANGES' PR reference.
Submitted by: ylavic
Reviewed by: ylavic, jchampion, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1802144 13f79535-47bb-0310-9956-ffa450edef68