PredicateBuilder should use in rather than eq for Arel::SelectManager

When using `where` against an arel subquery, ActiveRecord::Relation::PredicateBuilder matches the subquery to the attribute using `Arel::Predications#eq`, rather than `Arel::Predications#in`. We should be using `in` instead.

The statement

    Author.where(id: Book.arel_table.project(:author_id))

currently generates the sql

    SELECT \"authors\".* FROM \"authors\" WHERE \"authors\".\"id\" = (SELECT author_id FROM \"books\")

which results in a postgres error if the subquery returns more than one row.

It should generate

    SELECT \"authors\".* FROM \"authors\" WHERE \"authors\".\"id\" IN (SELECT author_id FROM \"books\")

instead.