KML viewer.flyTo placemark goes underground

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/E27D7WJHWZ8
Using viewer.flyTo for a pin created by this KML file results in the camera going under ground.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');
var options = {
camera : viewer.scene.camera,
canvas : viewer.scene.canvas,
clampToGround: true
};
var zoomTarget;

var kmlFile = "https://raw.githubusercontent.com/googlemaps/kml-samples/gh-pages/kml/Placemark/placemark.kml";
var promise = Cesium.KmlDataSource.load(kmlFile, options);
Cesium.when(promise, function(dataSource) {
viewer.dataSources.add(dataSource);
zoomTarget = dataSource.entities.values[0];
});

Sandcastle.addToolbarButton('Fly to Placemark', function() {
viewer.flyTo(promise);
}, 'toolbar');
```

Interestingly enough, the same thing does not happen when you create a pin at the same location with the entities API

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

var pinBuilder = new Cesium.PinBuilder();
var entity = viewer.entities.add({
position : Cesium.Cartesian3.fromDegrees(-122.087461,37.422069),
label : {
text : 'pin',
verticalOrigin : Cesium.VerticalOrigin.TOP
},
billboard : {
image : pinBuilder.fromColor(Cesium.Color.SALMON, 24).toDataURL(),
verticalOrigin : Cesium.VerticalOrigin.BOTTOM,
}
});

Sandcastle.addToolbarButton('Fly To Entity', function() {
viewer.flyTo(entity);
}, 'toolbar');
```

Start by reproducing the supplied Sandcastle example, then trace the KmlDataSource.load and viewer.flyTo entry points shown in the report. Compare the KML placemark camera destination with the entities API case; the issue is done when flying to the KML placemark no longer places the camera underground.

Written by the indexing model from the issue text.