In this case the longitude values are in the range [0,360]. We could create a useful output if the cube was rolled. I considered doing this in the test, but in the long term this will be completed using the proposed bounding box extract https://github.com/SciTools/iris/issues/77 .

A user could go ahead and write a GeoTiff with the [0,360] range if they wished. The image would be a valid geotiff, but values would fall outside of the plate caree projection when drawn by most GIS.

Currently the proposed constraint seems sufficient and desirable for the purpose of this unit test.