Allow ApiResponse.reference to be set as Schema.$ref in json and yaml

[jonkeda]

Hi Chen Kong,

Would you please have look at the changes I made? And consider if you can merge them?

It is meant to render the "reference" of an apiresponse to the json as a $ref field.

For instance in this example I added a reference:

_@ApiResponses(value = {@ApiResponse(code = 400, message = "Invalid pet value", reference = "https://pet.error.com")})_

And it is rendered as following:

    _"responses": {
      "400": {
        "description": "Invalid pet value",
          "schema" : {
              "$ref" : "https://pet.error.com"
          }
      }
    },_

I made the changes in the reader and changed one endpoint to test. All affected unit tests have been modified.

Kind regards,
Daan Jonkers