moment should work on non-square

This is a follow-on from #912 - there, in the process of fixing something *else*, @pllim incidentally uncovered an odd test case:
https://github.com/astropy/specutils/pull/912/commits/703affd3ca69f630d13718698327da67ce7aa4e8#diff-17f72e2d3ef7080f564b5e8468fabad5155bed36eba4f3ee531c2fee6ac1b6aaL1034

Specifically, if you do moment(order=2,axis=1) on a 2 (spatial)d Spectrum1d, it doesn't work if the spatial dimensions are not square. This seems wrong to me, as I would expect the moment along a single spatial direction to not care at all about the other dimensionalities. So the underlying problem should probably be tracked down.

Reproduce moment(order=2, axis=1) on a two-dimensional Spectrum1d with non-square spatial dimensions, then inspect the referenced PR #912 diff and the moment implementation to locate the dimensionality assumption. Done means the moment calculation works for rectangular spatial dimensions and a regression test covers that case.

Written by the indexing model from the issue text.