Fixes body.body in both physics plugins

[Burstaholic]

Seems this one was missed in an update.

[ghost]

Just wondering, how tested is this? I attempted to use the OIMO plugin, and it almost immediately crashed with this:

TypeError: Cannot read property 'next' of undefined
    at OimoJSPlugin.runOneStep (file:///lib/babylon.js:19357:36)
    at PhysicsEngine._runOneStep (file:///lib/babylon.js:19417:33)
    at Scene.render (file:///lib/babylon.js:11021:37)

The only way to fix it was to convert this.registeredMeshes[i].body to this.registeredMeshes[i].body.body twice in the code, which is a regression.

EDIT: CannonJS plugins appears to work properly though.

[Burstaholic]

I did not test the OIMO plugin, I just assumed it needed the same fix. My bad, I'll fix the pull request.

[deltakosh]

Hey I'll close this one and just update cannon.js :)