Corrected behavior of `extensibleMatch` filter (RFC4511 section 4.5.1).

See https://tools.ietf.org/html/rfc4511#section-4.5.1
Added tests to make sure matching using DN attributes behaves as expected.

Both match and no-match cases tested.
Corrected typo in comments; changed indentation.
Merge pull request #31 from cwaldbieser/dnfilter

BUGFIX: Extensible Match Filter based on DN attributes not working.