3D Character Controller choppy/stuck on kinematic rigidbodies

> Closed by #625

I'm using the same setup as the [3d character controller example](https://github.com/dimforge/bevy_rapier/blob/master/bevy_rapier3d/examples/character_controller3.rs) I made with *one* difference:

The ground cube has a `Rigidbody::KinematicPositionBased` component (line [74](https://github.com/dimforge/bevy_rapier/blob/3d0e4c4be83b73fb04bc94e3464a7995364b3b94/bevy_rapier3d/examples/character_controller3.rs#L74)).

```diff
commands.spawn((
Transform::from_xyz(0.0, -ground_height, 0.0),
Collider::cuboid(ground_size, ground_height, ground_size),
+ RigidBody::KinematicPositionBased,
));
```

Suddenly the player controller very choppy, getting stuck randomly on the ground, even though it's not moving.

I tried making a "Ground" and "Player" group and disable contact forces via `SolverGroups` without success.

I think this is a regression, as this setup used to work in previous versions. The only issue back then was that the character controller could get stuck on walls, but not on the ground.