Fix count(:all) with eager loading and explicit select and order

[kamipo]

This follows up ebc09ed.

We've still experienced a regression for size (count(:all)) with
eager loading and explicit select and order when upgrading Rails to 5.1.

In that case, the eager loading enforces distinct to subselect but
still keep the custom select, it would cause the ORDER BY with DISTINCT
issue.

% ARCONN=postgresql bundle exec ruby -w -Itest test/cases/relations_test.rb -n test_size_with_eager_loading_and_custom_select_and_order
Using postgresql
Run options: -n test_size_with_eager_loading_and_custom_select_and_order --seed 8356

# Running:

E

Error:
RelationTest#test_size_with_eager_loading_and_custom_select_and_order:
ActiveRecord::StatementInvalid: PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ..." ON "comments"."post_id" = "posts"."id" ORDER BY comments.i...
                                                             ^

As another problem on distinct is enforced, the result of count
becomes fewer than expected if select is given explicitly.

e.g.

Post.select(:type).count
# => 11

Post.select(:type).distinct.count
# => 3

As long as distinct is enforced, we need to care to keep the result of
count.

This fixes both the count with eager loading problems.