Keep model names after fitting

[xuanxu]

This PR makes fit_lines to preserve the names of the original model(s) after fitting.

Fixes #503

[nmearl]

Thanks @xuanxu! Can you please include a test (or extend an existing one) that checks that the names within compound models are also maintained (as shown in #503)?

[xuanxu]

@nmearl the PR already includes tests:

• for when there's a name: https://github.com/astropy/specutils/pull/526/files#diff-7fc9812436fbe451ddab32a963e73729R464-R471
• and for the case of models with empty name: https://github.com/astropy/specutils/pull/526/files#diff-7fc9812436fbe451ddab32a963e73729R479

[nmearl]

@xuanxu Those tests only pertain to a single model, not compound models as in the example in the issue.

[xuanxu]

@nmearl done! Compound models are covered now.

[nmearl]

Looks good, thanks @xuanxu!