HTTPASYNC-81: ensure request produce is closed in case of a premature exchange termination
Contributed by Markus Kull <markus.kull at 1und1.de>

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1616438 13f79535-47bb-0310-9956-ffa450edef68