Heightfield collision events not triggered when scale.y is set to -1

[lazystitan]

Description: When using Collider::heightfield with a negative Y-axis scale (e.g., Vec3::new(9., -1., 9.)), collision events are not triggered. However, setting scale.y to a positive value (e.g., 1.0) works as expected. This raises two questions:

1. Is this behavior intentional, or is it a bug?
2. What is the semantic meaning of the Y-component in the heightfield's scale parameter? Does a negative value invert the collision normal or flip the collision surface? Reproduction Steps:
3. Create a heightfield collider with scale.y = -1.0.
4. Add a dynamic rigid body above the heightfield.
5. Observe that no collision events are triggered when the body falls onto the heightfield.
6. Change scale.y to a positive value (e.g., 1.0).
7. Collision events are now triggered as expected. Code Example:

// No collision events when scale.y = -1
commands.spawn((
    Transform::from_xyz(0f32, 0f32, 0f32),
    RigidBody::Fixed,
)).with_child((
    Collider::heightfield(heights, noise_width, noise_long, Vec3::new(9., -1., 9.)),
    Transform::from_isometry(
        Isometry3d::new(
            Vec3::new(4.5, 0., 4.5),
            Quat::from_axis_angle(Vec3::new(1.0, 0., 1.0).normalize(), PI),
        )
    ),
));
// Collision events work when scale.y = 1
commands.spawn((
    Transform::from_xyz(0f32, 0f32, 0f32),
    RigidBody::Fixed,
)).with_child((
    Collider::heightfield(heights, noise_width, noise_long, Vec3::new(9., 1., 9.)),
    Transform::from_isometry(
        Isometry3d::new(
            Vec3::new(4.5, 0., 4.5),
            Quat::from_axis_angle(Vec3::new(1.0, 0., 1.0).normalize(), PI),
        )
    ),
));

Expected Behavior: Collision events should trigger regardless of the sign of scale.y, or the documentation should explicitly state if negative scaling is unsupported for heightfields.