Fix incomplete log message

Commit 48236b from 2014 introduced a logging improvement to avoid
logging each removed session per line and instead log one line at
the end with all removed sessions ids. However that list of removed
session ids wasn't populated. This commit fixes that.

Issue: SPR-14111

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/TransportHandlingSockJsService.java

@@ -359,14 +359,15 @@ public class TransportHandlingSockJsService extends AbstractSockJsService implem
 			if (this.sessionCleanupTask != null) {
 				return;
 			}
-			final List<String> removedSessionIds = new ArrayList<String>();
 			this.sessionCleanupTask = getTaskScheduler().scheduleAtFixedRate(new Runnable() {
 				@Override
 				public void run() {
+					List<String> removedIds = new ArrayList<String>();
 					for (SockJsSession session : sessions.values()) {
 						try {
 							if (session.getTimeSinceLastActive() > getDisconnectDelay()) {
 								sessions.remove(session.getId());
+								removedIds.add(session.getId());
 								session.close();
 							}
 						}
@@ -375,9 +376,8 @@ public class TransportHandlingSockJsService extends AbstractSockJsService implem
 							logger.debug("Failed to close " + session, ex);
 						}
 					}
-					if (logger.isDebugEnabled() && !removedSessionIds.isEmpty()) {
-						logger.debug("Closed " + removedSessionIds.size() + " sessions " + removedSessionIds);
-						removedSessionIds.clear();
+					if (logger.isDebugEnabled() && !removedIds.isEmpty()) {
+						logger.debug("Closed " + removedIds.size() + " sessions: " + removedIds);
 					}
 				}
 			}, getDisconnectDelay());