@Override
            protected String getUserDnSuffix() {
                return null;
            }
This test simulates that if a non-String principal (i.e. not a username) is passed as the LDAP principal, that
it is not altered into a User DN and is passed as-is.  This will allow principals to be things like X.509
certificates as well instead of only strings.

@throws NamingException not thrown
core/src/test/java/org/apache/shiro/realm/ldap/DefaultLdapRealmTest.java