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).