[R] Array JSON encoding doesn't work

Description

Cannot encode arrays in models to JSON.

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:
      nums:
        type: array
        items:
          type: number
      strs:
        type: array
        items:
          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$new(nums=list(1, 2, 3))$toJSONString()
Error: is.character(x) is not TRUE
> Pet$new(strs=list('foo', 'bar'))$toJSONString()
character(0)

Related issues/PRs

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

Suggest a fix/enhancement

This generated code is clearly wrong and should be fixed:

    toJSONString = function() {
       sprintf(
        '{
           "nums": [%s],
           "strs": [%s]
        }',
        lapply(self$`nums`, function(x) paste(paste0('"', x, '"'), sep=",")),
        lapply(self$`strs`, function(x) paste(paste0('"', x, '"'), sep=","))
      )
    },

Firstly, paste0 doesn't work on non-strings. Next, the paste with "sep" should be outside the lapply.

Reproduce the problem with the supplied Swagger declaration, the R generator command, and the generated Pet model's toJSONString() method. Trace the R generator template or entry point that emits array serialization, then verify that numeric and string arrays produce valid JSON without the reported error.