Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
So, I've had look into the code more carefully: The 'MeV' is just specific to this test, as it's not scale independent. It fails with 'GeV' where it didn't before because of the previous bug fix (before this, it assumed the 'GeV' energy was 'MeV' - and I used 'MeV' as the internal assumption because this is the energy unit usually used by cubes produced by the Fermi science tools')