Grouped query performance of total_count using Arel

Improved performance for total_count on grouped ActiveRecord::Relation.

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

kaminari-activerecord/lib/kaminari/activerecord/active_record_relation_methods.rb

@@ -32,15 +32,13 @@ def total_count(column_name = :all, _options = nil) #:nodoc:
 
       c = c.limit(max_pages * limit_value) if max_pages && max_pages.respond_to?(:*)
 
-      # .group returns an OrderedHash that responds to #count
-      c = c.count(column_name)
-      @total_count = if c.is_a?(Hash) || c.is_a?(ActiveSupport::OrderedHash)
-                       c.count
-                     elsif c.respond_to? :count
-                       c.count(column_name)
-                     else
-                       c
-                     end
+      # Handle grouping with a subquery
+      @total_count = if c.group_values.any?
+        sq = c.except(:select).select("1 AS record").arel.as("subquery")
+        c.model.connection.select_value Arel::SelectManager.new.from(sq).project(sq[:record].count)
+      else
+        c.count(column_name)
+      end
     end
 
     # Turn this Relation to a "without count mode" Relation.