Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
The fact that your tests passed on travis-ci tells you that this is not properly tested yet.
And I don't understand where we have the internal assumption that energy needs to be `GeV` ... please track this down and add a comment (or even better ... do the unit conversion at the point where it's really needed instead).