Support more options for openapi3 responses

[ngoral]

This request adds missing support for parsing responses with the same status code, but different content types. E.g.:

...
responses:
  '200':
    description: Foo
    content:
      application/json:
        ...
      text/plain:
        ...

It also adds good support for using $ref in a responses like this:

...
responses:
  '404':
    $ref: '#/components/responses/404NotFound'

...

components:
  responses:
    404NotFound:
      description: ...
      content: ...

And also add support for responses without payloads:

...
responses:
  '404':
    description: Not found

[ngoral]

@tuwilof I made a small refactoring, hope it doesn't bother you.

Now tests are failing because more info has appeared about responses that use references (for example 404s ones). But I think now it is working right because there was information missing: there was no schema.
If replacing application/json with application/vnd.api+json does not seem right, I can stub this and some more cases if you can see them now.

What do you think?

[tuwilof]

Thanks, this refactoring is good.

Yes, now the work is going right.

You can change the speed.json test file so that the test doesn't fail, and then I'll merge PR.

[ngoral]

Ok, I'll change it.

Only seems that it breaks compatibility...
I would like to pull a new version in fitting after this PR is accepted and new version of gem is out...

[tuwilof]

I have published your changes in the new version of the tomograph 3.2.0

[ngoral]

Thank you so much for being so cooperative!