Have Spectrum1D(spectrum,...) constructor copy a spectrum with individual attributes set.

#600 adds an example of spectral shifting by creating a new `Spectrum1D` object. As it stands this makes sense, but the method shown there has the flow that it requires the user to specify everything the want to copy over manually. So if e.g. there's an ``uncertainty``, or a ``meta`` or a ``mask``, the user has to remember to include them by hand.

Instead, I think it would be better if this worked:
```
>>> original_spectrum = ...
>>> shifted_spectrum = Spectrum1D(original_spectrum, spectral_axis=original_spectrum + shift)
```
i.e., the ``Spectrum1D(spec, ...something else..)`` construct would be understood to copy everything in ``spec`` *except* anything explicitly overridden in ``something else``.

Right now ``Spectrum1D(original_spectrum, spectral_axis=original_spectrum + shift)`` works, but silently fails to update the ``spectral_axis``. So I think this may just be a bug that needs fixing... but once it's fixed the example in #600 should be updated to reflect this idiom instead of explicitly copying the spectral axis.

Start by tracing the Spectrum1D constructor behavior described in this issue and review the spectral-shifting example from #600. Verify the constructor copies unspecified attributes such as uncertainty, meta, and mask while honoring an explicitly provided spectral_axis. Update the #600 example to use this constructor idiom and add or adjust coverage for the resulting behavior.

Written by the indexing model from the issue text.