fix for AD servers (we have not to follow anyway via connection options)

Hello, with this trivial patch ldapdb can work with AD servers that respond with referrals such as:

>> ldapConn.search_s(...)
[(None, ['ldap://ForestDnsZones.xx/DC=ForestDnsZones,DC=xx']),
 (None, ['ldap://DomainDnsZones.xx/DC=DomainDnsZones,DC=xx']),
 (None, ['ldap://xx/CN=Configuration,DC=xx'])]

Hope this helps, ciao