Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
I still think Travis CI skips these tests:

`gammapy/data/tests/test_spectral_cube.py ssssssssssssss`

There is also a `scipy` dependency on them (due to interpolation I think), so if `reproject` is OK, does Travis CI skip `scipy` tests?