change autosave_association.rb so that association autosave can work

[ypxing]

Without patching autosave_association.rb, below case will fail with error
TypeError: [:dorm_id, :room_id] is not a symbol nor a string

    room = Room.new(dorm_id: 1000, room_id: 1001)
    room_assignment = RoomAssignment.new(student_id: 1000)
    room_assignment.room = room
    room_assignment.save

The change is to fix the issue by using record.read_attribute instead of record.send in autosave_association.rb.

[cfis]

Thanks!