Default attribute for Active Directory password expiry is missing in LDAP authentication plugin.

When we added password expiry support for AD to the LDAP authentication plugin in 1.8, we forgot to add the default attribute for this functionality to the $default array in ldap_getdefaults(). This means that the administration must specify the right value in the LDAP settings page, or otherwise password expiry won't work (contrary to what other LDAP backends do, and what MoodleDocs say).

The attached patches fix the issue for 1.8, 1.9 and HEAD.

Saludos. Iñaki.