Untyped schemas aren't accepted

[gibson042]

Root cause of swagger-api/swagger-codegen#6896

Description

Parsing seems to require a type property despite it being optional in the specification (where it defines values that can be of any type).

Swagger declaration file content or url

swagger: '2.0'
info:
  title: 'map of arbitrary values'
  version: '1'
produces: ["application/json"]
paths:
  /mapOfAnything:
    get:
      operationId: mapOfAnything
      responses:
        default:
          schema:
            description: 'map of arbitrary values'
            type: object
            additionalProperties:
              description: Any JSON type

Suggest a fix/enhancement

Introduce necessary code for handling untyped schemas.