[Fix #332] Fix false negative for Rails/ReflectionClassName

[bubaflub]

This PR fixes a false negative for Rails/ReflectionClassName when the
relation had a scope parameter. The cop now considers relations with or
without the second optional scope parameter.

Before:

The following relation would not raise an offense even though the class_name parameter is a constant:

belongs_to :account, -> { distinct }, class_name: Account

After:

The relations with an optional second scope parameter like the example above are now considered by the cop.

---

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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• 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.

[bubaflub]

I tried my best to follow CONTRIBUTING.md, the pull request template, existing coding style, and existing git commit message style. If there's anything missing or incorrect please let me know and I'll fix it.

[bubaflub]

Would you like to see more test cases? If so, any thing in particular you'd like to see? I didn't think it was necessary to test belongs_to vs has_many as that's just part of the matcher syntax but I could write up those test cases if that'd be beneficial.

[koic]

Yeah, I think it can be accepted as the minimum case.

[bubaflub]

Should I have updated the docs to note that Rails/ReflectionClassName has been updated in the next version?

[koic]

Nice! I think this PR has won the solution to the issue. And I don't think the documentation needs to be updated because the focus of this cop issue is to class_name. Thank you!