HTTP2: SocketsHttpHandler doesn't downgrade to HTTP/1.1 during Windows authentication

When SocketsHttpHandler connects to a server using HTTP/2, it is unable to downgrade to HTTP/1.1 when the server requests Windows authentication (Negotiate or NTLM).  The net result is that 401 is returned to the client without even trying to reconnect (with HTTP/1.1) and send credentials.

```c#
// Repro will be posted later
```