ol.source.WMTS now passes tileClass option to ol.source.TileImage

The constructor for ol.source.TileImage accepts an option for the tileClass,
but ol.source.WMTS does not pass this to the parent. This pull request adds
that in.
Merge pull request #2838 from mike-mcgann/wmts-tile-class

ol.source.WMTS now passes tileClass option to ol.source.TileImage