Handle PONG messages in WebSocket handlers

See gh-534

spring-graphql/src/main/java/org/springframework/graphql/server/webflux/GraphQlWebSocketHandler.java

@@ -183,6 +183,9 @@ public class GraphQlWebSocketHandler implements WebSocketHandler {
 				case PING -> {
 					return Flux.just(this.codecDelegate.encode(session, GraphQlWebSocketMessage.pong(null)));
 				}
+				case PONG -> {
+					return Flux.empty();
+				}
 				case COMPLETE -> {
 					if (id != null) {
 						Subscription subscription = subscriptions.remove(id);

spring-graphql/src/main/java/org/springframework/graphql/server/webmvc/GraphQlWebSocketHandler.java

@@ -248,6 +248,8 @@ public class GraphQlWebSocketHandler extends TextWebSocketHandler implements Sub
 							ExceptionWebSocketHandlerDecorator.tryCloseWithError(session, ex, logger);
 						}
 					});
+			case PONG -> {
+			}
 			case COMPLETE -> {
 				if (id != null) {
 					Subscription subscription = state.getSubscriptions().remove(id);