octree dynamicContent and mesh.dispose

Hi, I was debugging using temporary meshes and noticed that if you just dispose the mesh after adding it to the octree.dynamicContent it stays there even after dispose.

Here's a playground where I create 500 meshes and then add them to octree.dynamicContent and then dispose of all of them and the dynamicContent keeps all 500.

http://babylonjs-playground.azurewebsites.net/#1YPN4G#2