Fixed binary compatibility breakage

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1515812 13f79535-47bb-0310-9956-ffa450edef68
HTTPCLIENT-1384: Expose CacheInvalidator interface
Contributed by Nicolas Richeton <nicolas.richeton at free.fr>

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1515814 13f79535-47bb-0310-9956-ffa450edef68