Reference `entity.dn` to get the string value

`entity[:dn]` returns an array with a single string element within. This
causes an issue with some LDAP implementations when passing that value
directly to the #auth method.

Instead we should use `entity.dn` which will return the first value
stringified and resolves the Connection Reset by peer issues.
Merge pull request #15 from localshred/fix_ldap_entity_dn_call

LDAP Authentication Failure: Connection reset by peer