AbstractHttpMessage to extend HeaderGroup

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1638359 13f79535-47bb-0310-9956-ffa450edef68
Blocking HttpEntity classes to implement HttpContentProducer interface; code clean in HttpEntity classes

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1638360 13f79535-47bb-0310-9956-ffa450edef68
Code cleanup in ContentType class

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1638361 13f79535-47bb-0310-9956-ffa450edef68
adding a callback to be executed when reusing an established connection from the pool

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1638390 13f79535-47bb-0310-9956-ffa450edef68