Layers not being displayed correctly when using WebMapTileServiceImageryProvider

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/bxVddKqNPRI

Data is displayed in the correct place initially, but then moves to the Northern Pole/Greenland region

Something similar may be happening in the [Sandcastle demo](http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Web%20Map%20Tile%20Service%20with%20Time.html&label=Beginner) as well

```
var provider = new Cesium.WebMapTileServiceImageryProvider({
url : 'https://mesonet.agron.iastate.edu/cache/tile.py/1.0.0/nexrad-n0q-900913/{TileMatrix}/{TileRow}/{TileCol}.png',
credit : new Cesium.Credit('Iowa Environmental Mesonet of Iowa State University')
});
```

Start by reproducing the issue with the WebMapTileServiceImageryProvider configuration in the report and compare it with the Web Map Tile Service with Time Sandcastle demo. Investigate why the imagery initially appears correctly and then moves toward Greenland; done means the layers remain correctly positioned after loading.

Written by the indexing model from the issue text.