New Post: Http digest preemptive authentication

As a client, I managed to authenticate to a server with digest authentication.
However, each of my requests are duplicated : first response has status 401, then the second request contains the required headers and the second response has status 200.
Keeping the same http_client object for all the following requests, I am surprised that the headers are not provided for the next requests, given that server nonce is constant (or changes seldomly).
This limitation is terrible in case of big POST (files uploads): each post is duplicated !

Found here that preemptive digest authentication does not seem to be supported (very surprising):
http://msdn.microsoft.com/en-us/library/windows/desktop/aa383144%28v=vs.85%29.aspx#supporting_authentication

(Tests performed with Win7 / msvc 2013 ; is it different with Win8 ?)

Any hint on how to solve that problem ?

Regards.