fix: Memory leak when using reactive subscriptions with KeepAlive

Author: KriKroff

graphql-kickstart-spring-webflux/src/main/java/graphql/kickstart/spring/webflux/ReactiveWebSocketSubscriptionSession.java

@@ -8,6 +8,7 @@
 public class ReactiveWebSocketSubscriptionSession extends DefaultSubscriptionSession {
 
   private final WebSocketSession webSocketSession;
+  private boolean opened = true;
 
   public ReactiveWebSocketSubscriptionSession(
       GraphQLSubscriptionMapper mapper, WebSocketSession webSocketSession) {
@@ -17,7 +18,7 @@ public ReactiveWebSocketSubscriptionSession(
 
   @Override
   public boolean isOpen() {
-    return true;
+    return opened;
   }
 
   @Override
@@ -34,4 +35,10 @@ public String getId() {
   public WebSocketSession unwrap() {
     return webSocketSession;
   }
+
+  @Override
+  public void close(final String reason) {
+    super.close(reason);
+    this.opened = false;
+  }
 }