mod_proxy: don't recyle backend announced "Connection: close" connections.

Failing to do this may lead to a race condition where we send a new request
before the backend really closes the connection (or lost SSL-Alert/FIN make
us think the connection is still alive, until the retransmission).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1703807 13f79535-47bb-0310-9956-ffa450edef68