plane collider never response to is_on_floor()

[dotquixote]

Godot version:
master

Issue description:
kinematic vs static plane
same condition, other collision mesh type has no problem.

Steps to reproduce:

extends KinematicBody

func _ready():
pass

func _physics_process(delta):
move_and_slide(Vector3(0,-1,0),Vector3(0,1,0))
if is_on_floor():
print("I am on Ground")

[akien-mga]

Duplicate of #5910.