make ZMQ_STREAM_NOTIFY also control disconnect notificatons

src/stream_engine.cpp

@@ -965,7 +965,7 @@ int zmq::stream_engine_t::write_subscription_msg (msg_t *msg_)
 
 void zmq::stream_engine_t::error (error_reason_t reason)
 {
-    if (options.raw_socket) {
+    if (options.raw_socket && options.raw_notify) {
         //  For raw sockets, send a final 0-length message to the application
         //  so that it knows the peer has been disconnected.
         msg_t terminator;