http2: set nextStreamID to 3 when AllowHTTP is set

Fixes golang/go#25230

Change-Id: Ie16295552fcd414555153626f62170ffb7bdba1d
Reviewed-on: https://go-review.googlesource.com/111835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/transport.go

@@ -567,6 +567,10 @@ func (t *Transport) newClientConn(c net.Conn, singleUse bool) (*ClientConn, erro
 	// henc in response to SETTINGS frames?
 	cc.henc = hpack.NewEncoder(&cc.hbuf)
 
+	if t.AllowHTTP {
+		cc.nextStreamID = 3
+	}
+
 	if cs, ok := c.(connectionStater); ok {
 		state := cs.ConnectionState()
 		cc.tlsState = &state