viewer.flyTo and viewer.zoomTo give different results in 2D and CV

`flyTo` always orients north, but `zoomTo` applies the specified heading. The doc seems to indicate this is by design (at least for 2D). But it also happens for CV. Why do it at all? I think `flyTo` should take the heading into account, just like `zoomTo` does.

While the below example uses the `viewer` commands, the actual behavior is defined in the `camera.flyToBoundingSphere` method.

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

var redRectangle = viewer.entities.add({
name : 'Red translucent rectangle with outline',
rectangle : {
coordinates : Cesium.Rectangle.fromDegrees(-110.0, 20.0, -80.0, 25.0),
material : Cesium.Color.RED.withAlpha(0.5),
outline : true,
outlineColor : Cesium.Color.RED
}
});

Sandcastle.addToolbarButton('zoomTo', function() {
viewer.zoomTo(redRectangle, new Cesium.HeadingPitchRange(Math.PI / 2, -Math.PI / 4, 4000000));
});

Sandcastle.addToolbarButton('flyTo', function() {
viewer.flyTo(redRectangle, {
offset : new Cesium.HeadingPitchRange(Math.PI / 2, -Math.PI / 4, 4000000)
});
});
```

Start with the provided viewer example and inspect the camera.flyToBoundingSphere method, comparing its behavior with zoomTo in 2D and CV. Done means the intended heading behavior is established and flyTo no longer gives a conflicting orientation where the issue expects it to match zoomTo.

Written by the indexing model from the issue text.