Improve check for "Broken pipe" error message

Issue: SPR-15802

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

@@ -355,8 +355,10 @@ public abstract class AbstractSockJsSession implements SockJsSession {
 	}
 
 	private boolean indicatesDisconnectedClient(Throwable ex)  {
-		return ("Broken pipe".equalsIgnoreCase(NestedExceptionUtils.getMostSpecificCause(ex).getMessage()) ||
-				DISCONNECTED_CLIENT_EXCEPTIONS.contains(ex.getClass().getSimpleName()));
+		String message = NestedExceptionUtils.getMostSpecificCause(ex).getMessage();
+		message = (message != null ? message.toLowerCase() : "");
+		String className = ex.getClass().getSimpleName();
+		return (message.contains("broken pipe") || DISCONNECTED_CLIENT_EXCEPTIONS.contains(className));
 	}