Memoize the result of Grape::Middleware::Base#response.

[Jack12816]

- What is it good for

When using the Grape::Middleware::Base#response method multiple times from within a custom middleware, the method generates a Rack::Response instance each (multiple) time.

Do not convert Rack::Response to Rack::Response was missing the memoization I think, because Don't create Grape::Request multiple times already added memoization for #request.

- What I did

While writing a simple middleware that should append a response header I hit an unexpected behavior while using the Grape::Middleware::Base#response helper. Example time:

class ResponseRoute < Grape::Middleware::Base
  # (1) Works with a Grape generated response,
  #     but not with a Rails generated response (looks like they aren't
  #     sharing the Rack::Utils::HeaderHash instance)
  def after
    response['X-Debug'] = 'true'
    response
  end

  # (2) Works with a Grape and Rails generated response 
  def after
    response.tap do |res|
      res['X-Debug'] = 'true'
    end
  end
end

After my fix, both versions will work as expected.

- A picture of a cute animal (not mandatory but encouraged)

[Jack12816]

@dblock could you add the hacktoberfest-accepted label? 🙃

[dblock]

@dblock could you add the hacktoberfest-accepted label? 🙃

What is this? :)

[Jack12816]

It's https://hacktoberfest.digitalocean.com/ - a monthlong "hackathon" where the people are encouraged to contribute to open-source projects. :)

Yeah, and I thought you could add Grape as a participating project or label my PR's individually to count for me :)

[dblock]

Done!

[Jack12816]

Awesome! Thank you ☺️