Use atomic variables instead of a lock

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/branches/4.3.x@1574547 13f79535-47bb-0310-9956-ffa450edef68
HTTPCLIENT-1474: fixed broken entity enclosing requests in HC Fluent

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/branches/4.3.x@1574548 13f79535-47bb-0310-9956-ffa450edef68
Minimal test coverage for HC Fluent

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/branches/4.3.x@1574550 13f79535-47bb-0310-9956-ffa450edef68