…stent keys of MockLDAP.directory.

 The implementation of the internal MockLDAP.directory dictionary is
 defined via

    self.directory = defaultdict(lambda: {})

 This results in a dictionary that always returns an empty dict for any
 key requested:

    >>> print (self.directory['some_key'])
    {}

 Thus, we cannot rely on the raisal of KeyError exceptions when accessing
 a non-existing LDAP DNs in the MockLDAP.directory.

 Instead, we check if MockLDAP.directory['some_key'] returns an empty
 dict {} value.

…und in the mock directory.

 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.