Includes HABTM returns correct size now. Fixes #16032 on rails 4-1-stable

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 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