OpenAPI router builder ignores $ref in responses

[mohamnag]

Version

v4.0.3, bug also present on current master branch

Context

In search of why the RouterBuilder does not add content type handler, I found out that operations' responses defined as $ref are totally ignored as the code (observable here) explicitly checks only for key content.

Do you have a reproducer?

Steps to reproduce

Consider following valid OpenAPI spec:

openapi: 3.0.1

paths:
  /some/path:
    get:
      summary: Some OP
      operationId: someOp
      responses:
        '200':
          $ref: '#/components/responses/SomeOpResponse'

components:

  responses:
    SomeOpResponse:
      description: OK
      content:
        application/json:
          schema:
            type: object
            properties:
              whatever:
                type: string

using RouterBuilder no ResponseContentTypeHandler will be mounted for operation someOp.