Spectrum1D WCS has wrong number of dimensions as an NDCube

As per title, it looks like that the dimensions of the WCS of Spectrum1D objects are not parsed correctly into the NDCube data class, as shown by the following snippet:

```python
import numpy as np
import astropy.units as u
from specutils import Spectrum1D
from astropy.modeling import models

wl = np.linspace(4000,12000,128)*u.angstrom
flux_1 = models.BlackBody(temperature=5800*u.K)(wl)
flux_2 = models.BlackBody(temperature=4800*u.K)(wl)
flux_3 = models.BlackBody(temperature=6800*u.K)(wl)
flux = np.array([flux_1, flux_2, flux_3])*flux_1.unit

spec = Spectrum1D(spectral_axis=wl, flux=flux)
print(spec.wcs.pixel_n_dim)
```

which returns

```
1
```

while it should be 3.

Run the supplied Python example and inspect how Spectrum1D constructs its WCS before it is exposed through the NDCube data class. Trace the reported pixel_n_dim value for the three-row flux array, then verify that the corrected result is 3 and add a regression check for this case.

Written by the indexing model from the issue text.