Check for collection docs and include them in generator

[mmenanno]

This resolves https://github.com/benbalter/jekyll-default-layout/issues/32

I grab first all the collection names, then I iterate through them pulling out their docs. These are then included in the flattened array that the existing documents method returns.

~This PR still needs tests added to it but seemed to be working when I tested locally on my example repo from my issue.~

[mmenanno]

I just added tests and in testing realised that posts are also considered a collection so I was able to remove the site.posts.docs call in the documents method since it was redundant and resulting in posts getting pulled into documents twice.

[mmenanno]

There were some rubocop violations unrelated to this PR that I fixed:

1. RSpec/SubjectDeclaration was mad about the let(:subject) use, instead of subject(:test_subject) (it also wasn't happy if I tried to do subject(:subject)
2. RSpec/NamedSubject was then mad after the above that I hadn't used the named subject test_subject instead of just calling subject
3. RSpec/SpecFilePathFormat was unhappy that the path was jekyll-default-layout instead of jekyll_default_layout.
4. RSpec/ContextWording was mad about a few context blocks that didn't start with when

[benbalter]

@halorrr I'm supportive of this functionality, but I suspect it will be a breaking change for many sites (that currently have collections rendered without a default layout). What do you think about making this feature opt-in, or potentially opt-in per collection? Also, what is the advantage of this approach vs. defining a per-collection default layout via frontmatter defaults?

[mmenanno]

@benbalter I didn't actually know about frontmatter defaults until you mentioned them, but reading about them I think this entire plugin could be replaced by using frontmatter defaults, so I'm not sure I can give a good reason for the advantage, other than awareness of options by the user base. Your plugin was referenced in the github pages docs which is what pointed me toward it.

I tested for my own usage of this plugin for my docs site was replicated with:

defaults:
  - values:
      layout: "default"

Which did apply to collections as well.

That being said, my only pushback to this being a opt-in feature, is that I don't really see any other config options in this plugin currently, so it would be a new pattern for this plugin. Might make more sense to just bump the major version number and add to the changelog about the new behaviour.