Camera.getRectangleCameraCoordinates doesn't account for terrain

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/R7CLnK9PZE8

In this example, the camera zooms to span where the rectangle would be on the WGS84 ellipsoid, which is zoomed in too far for where the rectangle is on the terrain surface.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
url : '//assets.agi.com/stk-terrain/world'
});

var west = -45.977340999999605;
var south = -21.417668000000273;
var east = -45.96351400000036;
var north = -21.403251000000008;
var rectangle = new Cesium.Rectangle.fromDegrees(west, south, east, north);
viewer.entities.add({
rectangle : {
coordinates: rectangle,
material: Cesium.Color.RED.withAlpha(0.5)
}
});

Sandcastle.addToolbarButton('Set View', function() {
viewer.camera.setView({
destination: rectangle
});
});
```

Start with Camera.getRectangleCameraCoordinates and reproduce the Sandcastle example using CesiumTerrainProvider and the supplied rectangle. Compare the camera framing for the WGS84 ellipsoid with the terrain surface; done means setting the rectangle view leaves the terrain-covered rectangle fully visible.

Written by the indexing model from the issue text.