Flush the output writer in case of SSE transport as well (#11955)

Change-Id: Id0d82eabab63681b9d01c0312588fdfe1659860f

server/src/com/vaadin/server/communication/PushHandler.java

@@ -331,9 +331,9 @@ public class PushHandler implements AtmosphereHandler {
             writer.write(event.getMessage().toString());
 
             switch (resource.transport()) {
-            case SSE:
             case WEBSOCKET:
                 break;
+            case SSE:
             case STREAMING:
                 writer.flush();
                 break;