posixGroup membership queries from Domain

`GitHub::Ldap::Domain#is_member?` doesn't correctly validate group membership for posixGroups due to some assumptions by the `Filter#member_filter` about the DN.

This changes the `Filter#member_filter` method to accept a `Net::LDAP::Entry` object and maps the correct group type with the correlated member attr (which, for posixGroup, is the member entry's CN).

Now we build the query with the appropriate membership attribute (`cn` for `memberUid`, as mapped above).

Added complexity/nesting here due to `entry["dn"]` and `entry["cn"]` both always returning an `Array` instead of a `String` like `entry.dn` did.