SEC-2820 MessageSecurityExpressionRoot class should be public

I work on a project with complex dynamic security access rules. We
implemented our own SecurityExpressionHandler and we want to instantiate
MessageSecurityExpressionRoot but we can't because it's package
protected.

messaging/src/main/java/org/springframework/security/messaging/access/expression/MessageSecurityExpressionRoot.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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. You may obtain a copy of
@@ -25,7 +25,7 @@ import org.springframework.security.core.Authentication;
  * @since 4.0
  * @author Rob Winch
  */
-final class MessageSecurityExpressionRoot extends SecurityExpressionRoot {
+public final class MessageSecurityExpressionRoot extends SecurityExpressionRoot {
 
     public final Message<?> message;