References to parameters and responses are not preserved

References to responses and parameters are replaced with the referenced object in the serialized output. It should be similar to definitions, which preserves references.

input.yaml

swagger: '2.0'
info:
  title: test
  version: "1.0.0"
  description: test
host: api.test.com
paths:
  '/buckets/{bucketKey}':
    get:
      description: test
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        '200':
          description: success
          schema:
            type: array
            items:
              $ref: './buckets.yaml'
    post:
      description: test
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        '200':
          $ref: '#/responses/objectKey'
  '/buckets/{bucketKey}/object':
    get:
      description: test
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        '200':
          $ref: '#/responses/objectKey'
parameters:
  bucketKey:
    name: bucketKey
    in: path
    description: Referenced parameter
    required: true
    type: string
responses:
  objectKey:
    description: Referenced response
    schema:
      type: object
      properties:
        paging:
          $ref: '#/definitions/StatusResponse'
definitions:
  StatusResponse:
    description: Referenced definition
    type: object
    properties:
      http_code:
        type: integer

buckets.yaml

---
description: "Definition from external doc"
type: "object"
properties:
  hello:
    type: "string"
  world:
    type: "integer"

output.yaml

swagger: "2.0"
info:
  description: "test"
  version: "1.0.0"
  title: "test"
host: "api.test.com"
paths:
  /buckets/{bucketKey}:
    get:
      description: "test"
      parameters:
      - name: "bucketKey"
        in: "path"
        description: "Referenced parameter"
        required: true
        type: "string"
      responses:
        200:
          description: "success"
          schema:
            type: "array"
            items:
              $ref: "#/definitions/buckets"
    post:
      description: "test"
      parameters:
      - name: "bucketKey"
        in: "path"
        description: "Referenced parameter"
        required: true
        type: "string"
      responses:
        200:
          description: "Referenced response"
          schema:
            type: "object"
            properties:
              paging:
                $ref: "#/definitions/StatusResponse"
  /buckets/{bucketKey}/object:
    get:
      description: "test"
      parameters:
      - name: "bucketKey"
        in: "path"
        description: "Referenced parameter"
        required: true
        type: "string"
      responses:
        200:
          description: "Referenced response"
          schema:
            type: "object"
            properties:
              paging:
                $ref: "#/definitions/StatusResponse"
definitions:
  StatusResponse:
    type: "object"
    properties:
      http_code:
        type: "integer"
    description: "Referenced definition"
  buckets:
    type: "object"
    properties:
      hello:
        type: "string"
      world:
        type: "integer"
    description: "Definition from external doc"
parameters:
  bucketKey:
    name: "bucketKey"
    in: "path"
    description: "Referenced parameter"
    required: true
    type: "string"
responses:
  objectKey:
    description: "Referenced response"
    schema:
      type: "object"
      properties:
        paging:
          $ref: "#/definitions/StatusResponse"

expected_output.yaml

swagger: "2.0"
info:
  description: "test"
  version: "1.0.0"
  title: "test"
host: "api.test.com"
paths:
  /buckets/{bucketKey}:
    get:
      description: "test"
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        200:
          description: "success"
          schema:
            type: "array"
            items:
              $ref: "#/definitions/buckets"
    post:
      description: "test"
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        200:
          $ref: '#/responses/objectKey'
  /buckets/{bucketKey}/object:
    get:
      description: "test"
      parameters:
      - $ref: '#/parameters/bucketKey'
      responses:
        200:
          $ref: '#/responses/objectKey'
definitions:
  StatusResponse:
    type: "object"
    properties:
      http_code:
        type: "integer"
    description: "Referenced definition"
  buckets:
    type: "object"
    properties:
      hello:
        type: "string"
      world:
        type: "integer"
    description: "Definition from external doc"
parameters:
  bucketKey:
    name: "bucketKey"
    in: "path"
    description: "Referenced parameter"
    required: true
    type: "string"
responses:
  objectKey:
    description: "Referenced response"
    schema:
      type: "object"
      properties:
        paging:
          $ref: "#/definitions/StatusResponse"

As you can see, the parameter-reference bucketEye and response-reference objectKey are referenced by multiple operations. In the output, those references are replaced by the referenced object's content. It results in duplicated content within the document. Also those references under parameters and responses sections are no longer used after serialization.

For definitions, the references are kept. If the reference is external then it's resolved and copied into the document and the new internal reference is used from that point on. Parameters and responses should at least keep the internal references intact.

A user would want to 'resolve' the external references, but still want to keep the internal references preserved. Ideally preserving the internal references should be the default behaviour, so the document doesn't get bloated by copying the referenced object's content to each instance.

Start with the serialization and reference-resolution path used for the input.yaml example; the issue provides input.yaml, buckets.yaml, output.yaml, and expected_output.yaml as comparison cases. Reproduce the serialized output and verify that internal parameter and response references remain as $ref entries while external references are resolved as shown.