Set MockSessionCookieConfig#maxAge default to -1

Issue: SPR-15142

spring-test/src/main/java/org/springframework/mock/web/MockSessionCookieConfig.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,7 +39,7 @@ public class MockSessionCookieConfig implements SessionCookieConfig {
 
 	private boolean secure;
 
-	private int maxAge;
+	private int maxAge = -1;
 
 
 	@Override

spring-web/src/test/java/org/springframework/mock/web/test/MockSessionCookieConfig.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,7 +39,7 @@ public class MockSessionCookieConfig implements SessionCookieConfig {
 
 	private boolean secure;
 
-	private int maxAge;
+	private int maxAge = -1;
 
 
 	@Override