WIP: Match DNs in a case-insensitive way, always use the object's DN as found in the mock directory.

[sunweaver]

LDAP DNs in LDAP servers are normally matched in a case-insensitive way. So for MockLDAP, we search our mock directory and try to find a case-insensitive match of the given DN.

Saying this, we always have to return the requested DN with the capitalization as stored in our mock LDAP directory. Otherwise, MockLDAP would behave differently from live LDAP server.

This PR includes the changes from PR #18. So, please, look at #18 first, merge it and then proceed to this PR.