MG_REQ_BEGIN -> MG_REQUEST, killed MG_REQ_END, add MG_REPLY. Restored server test

examples/websocket.c

@@ -37,7 +37,7 @@ static int send_reply(struct mg_connection *conn) {
 }
 
 static int ev_handler(struct mg_connection *conn, enum mg_event ev) {
-  if (ev == MG_REQ_BEGIN) {
+  if (ev == MG_REQUEST) {
     return send_reply(conn);
   } else if (ev == MG_AUTH) {
     return MG_TRUE;