AuthenticatedUser: Add final keyword for class members

Change-Id: Iaf1ded51cf100559c72593e6cca8af556f9267c0

src/main/java/com/thesamet/gerrit/plugins/saml/AuthenticatedUser.java

diff --git a/src/main/java/com/thesamet/gerrit/plugins/saml/AuthenticatedUser.java b/src/main/java/com/thesamet/gerrit/plugins/saml/AuthenticatedUser.java
index 3eae810..02583b3 100644
--- a/src/main/java/com/thesamet/gerrit/plugins/saml/AuthenticatedUser.java
+++ b/src/main/java/com/thesamet/gerrit/plugins/saml/AuthenticatedUser.java
@@ -17,10 +17,10 @@
 public class AuthenticatedUser implements java.io.Serializable {
   private static final long serialVersionUID = 1L;
 
-  private String username;
-  private String displayName;
-  private String email;
-  private String externalId;
+  private final String username;
+  private final String displayName;
+  private final String email;
+  private final String externalId;
 
   public AuthenticatedUser(String username, String displayName, String email, String externalId) {
     this.username = username;