Added a spec that ensures that getPickRay returns undefined if the canvas is not fully rendered
Added a spec that ensures that getPickRay returns undefined if the canvas is not fully rendered

likangning93/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium