This follows up ebc09ed9.

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.

```ruby
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.