Unexpected camera rotation with lookAt in 2D mode.

The camera rolls 180 degrees after calling lookAt in 2D mode. Is this expected?

Here's a sandcastle example: http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=ce7747d6784fe914c5d8b249a20ed282

Start by reproducing the linked Sandcastle example in 2D mode and verify the 180-degree roll after calling lookAt. Then trace the camera lookAt behavior for 2D mode and determine whether the rotation is expected; done means documenting the intended behavior or fixing it with a regression test.

Written by the indexing model from the issue text.