Check memberships recursively

Looks like we're not properly handling recursive group membership checks. Opening this PR with a failing test to set the state for a fix.

This is necessary to properly partition groups from members here: https://github.com/github/github-ldap/blob/744b141a0f86961be5453984b31789a8285995d2/lib/github/ldap/group.rb#L109-L114

As documented, `Net::LDAP` downcases object classes (et al) by default.