HTTPCLIENT-1441: Caching AsynchronousValidationRequest leaks connections.
Contributed by Dominic Tootell <dominic.tootell at gmail.com>

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1546855 13f79535-47bb-0310-9956-ffa450edef68