Implements additional unit tests for integral_flux_image in spectral_cub...

...e

Adds the unit test changes suggested in PR #178 comment.
Reduce this repetetive code a bit, e.g.
```
expected = [('CDELT1', 0.5),
                    ...
                  ]
for key, value in expected:
    assert_allclose(np.abs(image.header[key]), value)
```