Add new Rails/UnusedRenderContent cop

[samrjenkins]

Issue: #967

The Rails documentation states:

If you try to render content along with a non-content status code (100-199, 204, 205, or 304), it will be dropped from the response.

https://guides.rubyonrails.org/layouts_and_rendering.html

On a number of occasions I have seen controller actions containing code such as:

render status: 204, plain: 'foo'

In cases such as these, the specified content for the response body is ignored and the code should be refactored to:

render status: 204

This pull request introduces a cop which identifies instances of unused body content.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Can you add Severity: warning because this looks like a Lint-equivalent rule.

+ Severity: warning

[koic]

# required.

Suggested change

	# This cop checks for uses of `render` which specify both body content and a non-content status.
	# @example
	# This cop checks for uses of `render` which specify both body content and a non-content status.
	#
	# @example

[samrjenkins]

Thanks for your feedback, @koic. I have addressed both comments. I also rebased onto latest master to address issues of failing tests. Let me know if you would like me to squash the new commit.

[koic]

@samrjenkins Yeah, looks good to me. Can you squash these commits into one?

[samrjenkins]

@koic done 👍

[koic]

Thanks!