Support delegated credentials (ISC_REQ_DELEGATE) via ClientWinAuth

If you use this Wikipedia graphic as a reference, http://en.wikipedia.org/wiki/Kerberos_(protocol)#mediaviewer/File:Kerberos.svg, the current implementation in HttpClient 4.4alpha1 does the first (red) and last (green) steps but it doesn't do the middle one (middle). By adding a parameter, it won't skip out on the middle step (where the Windows LSA will ask the Windows domain controller to generate a ticket-granting-ticket for the requested service).

In WindowsNegotiateScheme.getToken(), the change would be to update Sspi.ISC_REQ_CONNECTION to Sspi.ISC_REQ_CONNECTION | Sspi.ISC_REQ_DELEGATE.