`find_in_batches` now returns an `Enumerator` when called without a block, so that it
can be chained with other `Enumerable` methods.
Merge pull request #13201 from marcandre/find_in_batch_enumerator

`find_in_batches` now returns an `Enumerator`

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/relation/batches.rb