[R] JSON serialization fails with object properties

Description

Attempting to JSON serialize a model containing a null property results in an R error, resulting in an empty string JSON output. Furthermore, it calls the wrong method (toJSON vs toJSONString), so non-nulls don't render properly.

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:
  Tag:
    type: "object"
  Pet:
    type: "object"
    properties:
      tag:
        $ref: "#/definitions/Tag"

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

$ R
> library(devtools)
> install(".")
> library(swagger)
> Pet$new()$toJSONString()
Error in sprintf("{\n           \"tag\": %s\n        }", self$tag$toJSON()) : 
  attempt to apply non-function

Suggest a fix/enhancement

• Null checks in JSON serialization.
• Call toJSONString() on object fields, not toJSON(), or refactor more heavily.

Reproduce the failure with the generated Pet model using Pet$new()$toJSONString(), then inspect the generated R serialization path and the object-field calls shown in the error. Done means null properties serialize without an R error and non-null object properties use the working JSON-string method so the output is populated.