Add more unit tests for spectral_cube.integral_flux_image

Adds the unit test changes suggested in PR #178 comment.
`expected` is a Quantity, however `actual` is not (because the ImageHDU returned by `integral_flux_image` no longer returned quantities).

`actual = array(1.8630308796498186e-07)`
`expected = <Quantity 1.8642901977727886e-07 1 / (cm2 s sr)>`