Includes HABTM returns correct size now. It's caused by the join dependency
only instantiates one HABTM object because the join table hasn't a primary key.

Fixes #16032.

Examples:

    before:

    Project.first.salaried_developers.size # => 3
    Project.includes(:salaried_developers).first.salaried_developers.size # => 1

    after:

    Project.first.salaried_developers.size # => 3
    Project.includes(:salaried_developers).first.salaried_developers.size # => 3