fix: memory leak due to ReactiveApolloSubscriptionSession not closing

A memory leak was possible because ReactiveApolloSubscriptionSession did
not call close on the ReactiveWebSocketSubscriptionSession that it
owned.

Author: tomas-c

graphql-kickstart-spring-webflux/src/main/java/graphql/kickstart/spring/webflux/apollo/ReactiveApolloSubscriptionSession.java

@@ -20,6 +20,12 @@ public boolean isOpen() {
     return session.isOpen();
   }
 
+  @Override
+  public void close(String reason) {
+    super.close(reason);
+    session.close(reason);
+  }
+
   @Override
   public Map<String, Object> getUserProperties() {
     return session.getUserProperties();