mod_proxy_http: Don't expect the backend to ack the "Connection: close" to
finally close those not meant to be kept alive by SetEnv proxy-nokeepalive
or force-proxy-request-1.0, and respond with 502 instead of 400 if its
Connection header is invalid.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1669558 13f79535-47bb-0310-9956-ffa450edef68