Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
Actually the idiom we should use if we need a given unit for some quantity is
```
energy_centers = Quantity(energy_centers, 'GeV')
```
but in this case please add a comment line why `'GeV'` is needed (is it the assumed internal unit in some part of the class)?