Don't send key with every Bing tile request.
Don't send key with every Bing tile request.

Key is only needed on the initial metadata request.

likangning93/cesium,NaderCHASER/cesium,progsung/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,oterral/cesium,jasonbeverage/cesium,jasonbeverage/cesium,CesiumGS/cesium,oterral/cesium,likangning93/cesium,YonatanKra/cesium,NaderCHASER/cesium,YonatanKra/cesium,likangning93/cesium,hodbauer/cesium,oterral/cesium,AnalyticalGraphicsInc/cesium,hodbauer/cesium,progsung/cesium,YonatanKra/cesium,josh-bernstein/cesium,CesiumGS/cesium,hodbauer/cesium,josh-bernstein/cesium,likangning93/cesium,CesiumGS/cesium,kaktus40/cesium,soceur/cesium,likangning93/cesium,YonatanKra/cesium,NaderCHASER/cesium,CesiumGS/cesium,geoscan/cesium,soceur/cesium,soceur/cesium,kaktus40/cesium,geoscan/cesium