- fixed a potential memory leak in Poco::Net::HTTPClientSession if it is misused
  (e.g., sendRequest() is sent two times in a row without an intermediate call to
  receiveResponse(), or by calling receiveResponse() two times in a row without
  an intermediate call to sendRequest()) - GH #217
- removed a few unnecessary protected accessor methods from Poco::Net::HTTPClientSession
  that would provide inappropriate access to internal state