Satisfy Rubocop's guard clause expectations.

Big thanks (づ｡◕‿‿◕｡)づ to @mikegee for the suggestion in
https://github.com/rspec/rspec-rails/commit/f6c277f8cb#commitcomment-7167216.
That provided the vast majority of the refactor.

At the end of the day, this changes makes a lot more sense, separating
concerns in a more logical location.
Merge pull request #1136 from rspec/make-rubocop-happy-with-http-status-matcher

Satisfy Rubocop's guard clause expectations.