KMLDataSource with network links reports BBOX of 0, 0, 0, 0 in 2D

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/mIEI2f8drkQ

If you zoom into the map in 2D and look in the Network tab at the xhr calls made, you'll see that around the point you're zoomed so a state fills the screen, the calls made have a bounding box of zero: `facilities.kml?BBOX=0,0,0,0`. If you zoom back out, the bounding box is correct.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {sceneMode: Cesium.SceneMode.SCENE2D});
var options = {
camera : viewer.scene.camera,
canvas : viewer.scene.canvas
};

Sandcastle.addToolbarButton('Load KML Network link',
function() {
var kmlText='' +
'' +
'0' +
'0' +
'' +
'onStop' +
'0' +
'BBOX=[bboxWest],[bboxSouth],[bboxEast],[bboxNorth]' +
'';
var dom = new DOMParser().parseFromString(kmlText, 'text/xml');
viewer.camera.flyHome(0);
viewer.dataSources.add(Cesium.KmlDataSource.load(dom, options));
});
```

Reproduce the issue with KmlDataSource.load using the provided Sandcastle example and SceneMode.SCENE2D, then inspect the Network tab for the generated NetworkLink requests. Trace how the camera view becomes the BBOX query parameters; done means zooming in no longer produces BBOX=0,0,0,0 while zoomed near a state.

Written by the indexing model from the issue text.