Ensure site.data.authors is properly formatted before attempting to retrieve author meta

[benbalter]

If site.data.authors is an array, or if site.data.authors[author] is anything other than a Hash, the build will error out.

This PR makes Drop#author_hash a bit more robust by ensuring site.data.authors and site.data.authors[author] is a hash before attempting to access it or add additional elements.

The user-facing functionality should be identical, but more robust.

This problem arises when Jekyll SEO Tag is included (by default in some cases) on GitHub Pages, and the user has a _data/authors.yml that was not formatted to work with Jekyll SEO tag.

[benbalter]

Failing test fixed via #228.

[DirtyF]

💯
Tested: No build error if authors is an array whether it's in the post Front Matter, the _config .yml or the authors.yaml data file. Excellent.

[benbalter]

Thanks for the 👀! 😄