Make Test IMAP Server fields volatile

`resetServer()` runs on a different thread.

Author: garyrussell

spring-integration-test-support/src/main/java/org/springframework/integration/test/mail/TestMailServer.java

@@ -215,9 +215,9 @@ else if ("QUIT".equals(line)) {
 
 	public static class ImapServer extends MailServer {
 
-		private boolean seen;
+		private volatile boolean seen;
 
-		private boolean idled;
+		private volatile boolean idled;
 
 		ImapServer(int port) throws IOException {
 			super(port);