[R] enum definitions are not usable

Description

Defined enums generate an empty class. This type is then enforced for methods and other models, making it unusable.

Swagger-codegen version

swagger-codegen-cli-2.4.0-20180821.200234-311

Swagger declaration file content or url

swagger: "2.0"
info:
  description: ""
  title: "repro"
  version: "1"
paths:
  /pet:
    post:
      tags:
      - "pet"
      summary: "Add a new pet to the store"
      description: ""
      operationId: "addPet"
      parameters:
      - in: "body"
        name: "body"
        description: "Pet object that needs to be added to the store"
        required: true
        schema:
          $ref: "#/definitions/Pet"
      responses:
        200:
          description: ""
definitions:
  Pet:
    type: "object"
    properties:
      status:
        $ref: "#/definitions/PetStatus"
  PetStatus:
    enum: [available, pending, sold]

Command line used for generation

java -jar swagger-codegen-cli-2.4.0-20180821.200234-311.jar generate --lang r -i swagger.yaml

Steps to reproduce

Try to create a new Pet using a PetStatus, cannot use enum values.

Pet$new("pending")

-> fails with wrong type

PetStatus is not usable as the generated class looks like this:

PetStatus <- R6::R6Class(
  'PetStatus',
  public = list(
    initialize = function(){
    },
    toJSON = function() {
      PetStatusObject <- list()

      PetStatusObject
    },
    fromJSON = function(PetStatusJson) {
      PetStatusObject <- jsonlite::fromJSON(PetStatusJson)
    },
    toJSONString = function() {
       sprintf(
        '{
        }',
      )
    },
    fromJSONString = function(PetStatusJson) {
      PetStatusObject <- jsonlite::fromJSON(PetStatusJson)
    }
  )
)

Suggest a fix/enhancement

1. Type enums (by reference) as strings.
2. or, generate enum values in the enum definition models

Start by reproducing the issue with the provided Swagger declaration and the java -jar swagger-codegen-cli-2.4.0-20180821.200234-311.jar generate --lang r command. Inspect the generated PetStatus and Pet R6 classes and the R generator templates; done means enum values can be passed to Pet$new and serialized correctly without the wrong-type failure.