Support descending order for in_batches without block.

[alpaca-tc]

Summary

Follow-up #30590.
In the feature of descending order for batches introduced in #30590 but the case of calling in_batches without a block was not supported.
Perhaps the reason it was not supported was a correction omission.

Other Information

When calling in_batches with block. It works fine.

# Post Pluck (0.1ms)  SELECT "posts"."id" FROM "posts" ORDER BY "posts"."id" DESC LIMIT ?  [["LIMIT", 1000]]
Post.in_batches(order: :desc) {}

When calling in_batches without block.

before: expected DESC, got ASC

# Post Pluck (0.1ms)  SELECT "posts"."id" FROM "posts" ORDER BY "posts"."id" ASC LIMIT ?  [["LIMIT", 1000]]
Post.in_batches(order: :desc).each {}

after

# Post Pluck (0.1ms)  SELECT "posts"."id" FROM "posts" ORDER BY "posts"."id" DESC LIMIT ?  [["LIMIT", 1000]]
Post.in_batches(order: :desc).each {}

[yahonda]

Would you explain why the order option is necessary without a block?
Unless a block is given, all of the records are handled in batches but the order does not look required.

[yahonda]

Ah, I understand that without an order specified it may lead to wrong results because each batched record set order is not guaranteed.

[yahonda]

Please disregard my previous two comments.

Regardless order by ... desc or order by ... asc without block lead to the same result because all of the batched results are identical, just the order is different.

but this behavior inconsistency may surprise users as you did. Let's merge this pull request.