This test's data was built using a hash keyed by a name which was
included twice, so it would always pass as long as total_count builds a
valid query. Fixed by using a 2D array instead.

Improved performance for total_count on grouped ActiveRecord::Relation.

This re-implements kaminari#979 using Arel to avoid issues kaminari#1012 and kaminari#1015

Using `Arel.star` so it does not depend on an alias set in the subquery,
as suggested by @guigs on kaminari#1022.

Only count non-null values of column_name if supplied. This meets the
semantics of the SQL count function without requiring that column_name
is one of the grouped columns.

Arel::SelectManager.new without args is unsupported in ActiveRecord < 5.