Add failing test showing mask band bug.
Add failing test showing mask band bug.

youngpm/rasterio,johanvdw/rasterio,kapadia/rasterio,brendan-ward/rasterio,njwilson23/rasterio,kapadia/rasterio,njwilson23/rasterio,brendan-ward/rasterio,youngpm/rasterio,perrygeo/rasterio,clembou/rasterio,brendan-ward/rasterio,clembou/rasterio,njwilson23/rasterio,johanvdw/rasterio,kapadia/rasterio,perrygeo/rasterio,perrygeo/rasterio,clembou/rasterio,youngpm/rasterio,johanvdw/rasterio