Implements additional unit tests for integral_flux_image in spectral_cub...

...e

Adds the unit test changes suggested in PR #178 comment.
I think this line is a no-op:
```
>>> from astropy.units import Quantity
>>> e = Quantity(10, 'TeV')
>>> e
<Quantity 10.0 TeV>
>>> e.to('GeV')
<Quantity 10000.0 GeV>
>>> e
<Quantity 10.0 TeV>
```
Why do you think it's a bug?
If it is, please add a test and fix it.