Nested merge now groups using a list of key combos

This means that when multiple group_by parameters are specified we can
group by all of them and include all of them in the same group of
results.
@nickgravgaard explained that it makes the nested merge easier https://github.com/alphagov/backdrop/pull/339/files#diff-2287dd59beed84f9be139886abdfa556R37

When we get rid of nested results we can have a flat list of keys.