Includes HABTM returns correct size now. Fixes #16032 and #15244 for master

    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.
    Updated commit and test fixed from @bigxiang commit dbaa837

    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
I think this is indicating a deeper problem with the preloader. I'm not sure we should be making a change like that without a full understanding of why it was needed in the first place.