Minitest/AssertEqual

[herwinw]

Prefer assert_equal over assert(expected == actual)
(I'll leave the updates to the changelog until the merge of #29, to save myself some merge conflicts. The remainder of the code can be reviewed)

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[abhaynikam]

@herwinw This PR would also need a changelog entry. Rest LGTM 🎉

[bbatsov]

Apart from the small remark I made the cop looks ok to me. You can add a changelog entry now.

[herwinw]

Fixed all the remarks, should be ready to merge

[bbatsov]

Thanks!