Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
At some point just the value is called - and I required this to be in GeV if I remember correctly (in some instances Quantities aren't supported - e.g ImageHDUs, so I couldn't find a way of avoiding this). If I don't do the conversion, the energy will just be passed in as a value without being convered to 'GeV' without this line.