[Fix #2898] Allow method definitions inside Class.new(SuperClass)

[segiddins]

Before submitting a PR make sure the following are checked:

• Wrote [good commit messages][3].
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not rebase)
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog accordingly. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[bbatsov]

👍