Implement smarter merge

[exoego]

Closes #48

This attempts to implement the behavior described in

everything under the path/method should be erased while leaving keys that rspec-openapi cannot update as is,
and then changes generated by rspec-openapi should be added to it.

[exoego]

This is a special treatment to avoid merging anything to Reference Object.
The reference object can have only $ref field.

# invalid 
  schema:
     $ref:  "#/components/schemas/Pet"
     description: "override"

# valid 
  schema:
     allOf:
      - $ref:  "#/components/schemas/Pet"
      - description: "override"

[k0kubun]

Is this for the case that $ref is manually added to an OpenAPI schema file? If you manually maintain the schema of the response, why don't you skip updating OpenAPI from such test cases? Do you have a test case that calls multiple endpoints and want to update OpenAPI for only one of these?

[k0kubun]

I wondered if just overwriting the value of base is fine, but given that what we have as an array in the current test in this repository are just servers, tags, parameters, and example, it's probably okay to rewrite it, ignoring the possibility of the need to partially update arrays inside it. We may need to recursively do something if we find other array fields that are not compatible with this idea, but I don't come up with such a thing at this moment.

[k0kubun]

Similar to https://github.com/k0kubun/rspec-openapi/pull/55/files#r877575816, as long as it updates only fields that rspec-openapi generates, it's probably fine, I guess.

[k0kubun]

This attempts to implement the behavior described in

everything under the path/method should be erased while leaving keys that rspec-openapi cannot update as is,
and then changes generated by rspec-openapi should be added to it.

I'm okay with merging this patch, but note that the behavior of your implementation is not what you quoted. This PR simply doesn't implement "everything under the path/method should be erased". For example, when an endpoint currently returns { foo: 1, bar: 2 } and it's changed to just { foo: 1 }, rspec-openapi should assume the bar field is removed from the API and remove it from the existing file. But this patch doesn't do it. It only updates fields and never erases anything. So this PR actually doesn't close #48.

Achieving it while still leaving legit manual changes isn't a simple task. It shouldn't be a simple recursive method, but it should be aware of which level of the schema it's currently at. That's why I called it "intelligent".

But at this point, I'm not comfortable leaving #48 open since nobody would understand my intention of the current description. I don't afford to put so much effort on explaining it with concrete examples either.

So, let's merge this and close #48. At least it brings us closer to what #48 says. Thanks.