Add/Remove Dates When Publishing/Unpublishing

[gavinhoward]

My second attempt to fix dates when publishing and unpublishing. The first attempt was #91, and it had several problems. I believe that this PR fixes the problems with #91 that I was told about; if not, please let me know.

I took the time to learn how to use git merge better, so after some work, I was able to keep @alzeih's commit (and authorship) intact.

I hope that it was okay to put myself as the author on the other commits, though.

Anyway, here are the changes I made:

1. I learned how to use RSpec and introduced the tests that were missing.
2. I fixed the RuboCop offenses listed by /script/fmt, though I did fix each one manually (not with /script/fmt -a).
3. I made sure that dates are completely removed when unpublishing a post.

This commit will close #55 and will also resolve #63.

@wip ready for review

[ashmaroli]

@gavinhoward Good Job with learning the new stuff.
The commits are proper this time around. However, I'll recommit the changes with you as co-author so that you'll get due credit when our merge-bot merges this PR. (It squashes all commits automatically prior to the merge.)

[gavinhoward]

@ashmaroli thank you. If it helps, I feel no particular need to be credited.

[ashmaroli]

Needs some changes to be made..

[ashmaroli]

LGTM! But would help to get a second opinion since the recent changes are my suggestions

[DirtyF]

Nice work @gavinhoward 👍

[DirtyF]

@jekyllbot: merge +minor

[DirtyF]

@gavinhoward 📆 shipped in /~https://github.com/jekyll/jekyll-compose/releases/tag/v0.11.0 🎉

[gavinhoward]

Yes! \o/