HTTPCORE-339: tolerate empty charset attribute when parsing content-type values

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1482742 13f79535-47bb-0310-9956-ffa450edef68
Added #onClose() event method to AbstractAsyncRequestConsumer and AbstractAsyncResponseConsumer

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1482743 13f79535-47bb-0310-9956-ffa450edef68