Reproduction Steps:
1. Open "Rigidbody2D.isKinematic Bug.zip" project
2. Open "Scene" scene
3. Play scene
4. Press "A" key to move "Rigidbody"
5. Press "D" key to change "Rigidbody.isKinematic" to "true"
6. Notice it stops
7. Press "LeftArrow" key to move "Rigidbody2d"
8. Press "RightArrow" key to change "Rigidbody2D.isKinematic" to "true"
9. Notice it does not stop

Expected behaviour: When Rigidbody2D is kinematic it removes all forces just as Rigidbody does.
Actual result: Rigidbody2D does not remove forces, it does not allow to add any more.

Reproduce on versions: Unity 5.5.0p3, 5.6.0b3
Did not reproduce on 5.4.4f1
Regression introduced in 5.5