Fixed Group.member_users and member_groups to work with empty groups

The Group.member_users and Group.member_groups methods would fail when
empty groups are present in the hierarchy.  Changed the access method
to [:member] to return an empty array instead.
Return only one copy of users/groups in member_users and member_groups methods

The Group.member_users and Group.member_groups methods would return
multiple copies of the same Net::LDAP record if the user/group existed
multiple times in the hierarchy.  Uniq’d the array to only return one
copy.