Local rectangle bug of TMS imagery provider has been fixed.

Possible fix for [TileMapServiceImageryProvider bug when using local rectangle](https://github.com/AnalyticalGraphicsInc/cesium/issues/2025).
`WebMercatorTilingScheme` doesn't take a `rectangle` parameter (though arguably it should).  That's the only reason that the Cesium logo TMS provider in the "Imagery Layers Manipulation" Sandcastle example isn't broken by this change.