Use correct service principle name (HTTP/hostname) for WinHttpClients

If we use WinHttpClients as is, HTTP Negotiate authentication won't work because the ticket being generated always uses 'null' as the service principle name (SPN).

According to the informational RFC 4559, the service principle name (SPN) should be in the following form: HTTP/hostname.

Users can work around it ourselves if we did something similar to http://stackoverflow.com/a/22865583 but it would be great if the sample code works out of the box