[R] strings are unquoted in JSON encoding

Description

toJSONString() produces invalid JSON when containing string values.

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:
      foo:
        type: string

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$new(foo="bar")$toJSONString()
[1] "{\n           \"foo\": bar\n        }"

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/8610

Suggest a fix/enhancement

Quote string values.

Start with the generated R model's toJSONString() path and reproduce the issue using Pet$new(foo="bar") from the report. Verify that string-valued fields are emitted as valid JSON, then add or update coverage for this example and confirm the generated output parses as JSON.