sync svn r23693 - [ZF-11002] ehancement implemented as proposed

This is just sync of r23693
I need re-review for this. (sorry, I'm not familiar with LDAP )

@see 
http://framework.zend.com/issues/browse/ZF-11002
https://github.com/zendframework/zf1/blob/master/library/Zend/Auth/Adapter/Ldap.php#L495
https://bitbucket.org/sasezaki/mirror-zf1-standard-trunk-libraray-auth-dir/commits/72cf4eba6ab675fca68a5bced4b5c3fd4a32c557 (commit-log-mirror)