Resolve RuboCop TODOs

[willnet]

ref: #395

Currently, the .rubocop_todo.yml file lists many cops as TODOs. There are still quite a few that can be fixed relatively safely with rubocop -a. I’d like to start clearing these.

https://github.com/Sorcery/sorcery/blob/939b7dc34f9a103327e464098268fc2494ce7568/.rubocop_todo.yml

To keep reviews simple, please avoid fixing multiple cop violations in a single commit. Aim to fix one cop per commit whenever possible.

I haven’t thoroughly reviewed the TODO list, so some items may be better disabled than resolved. If you find a cop that should be disabled, please update .rubocop.yml with the change and include the reason.

[beengine]

Hi @willnet, can I work on this issue?

[willnet]

Thanks, @beengine ! Yes, please go ahead.

[beengine]

@willnet could you please take a look at my PR? At least to check if I'm on the right track?