Fix PredicateBuilder::ArrayHandler so that it uses the correct Arel pred...

...icate when given a nested array

Fixes #16580

PredicateBuilder::ArrayHandler was using the incorrect Arel predicate (Arel::Nodes::Equality) when given a nested array. This fix restores the behavior on 4.1 stable and will utilize Arel::Nodes::In when given a nested array.
Is there a reason for using `assert_sql` here, over, say, `assert_equal 2, Post.where(title: [titles]).count`?