Fix undertow access_log file name

Undertow 1.3.2 changed the default access log file suffix from '.log'
to just 'log'. Thus we need to adapt the file name pattern to include
the missing dot.

Closes gh-4670

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainerFactory.java

@@ -384,7 +384,7 @@ public class UndertowEmbeddedServletContainerFactory
 		try {
 			createAccessLogDirectoryIfNecessary();
 			AccessLogReceiver accessLogReceiver = new DefaultAccessLogReceiver(
-					createWorker(), this.accessLogDirectory, "access_log");
+					createWorker(), this.accessLogDirectory, "access_log.");
 			String formatString = (this.accessLogPattern != null) ? this.accessLogPattern
 					: "common";
 			return new AccessLogHandler(handler, accessLogReceiver, formatString,