[cpprest] Codegen generates class with invalid syntax code causing compilation failure

Description

The cpprest codegen generates the Client code with invalid syntax for API definition which contains list of strings as the body content.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

swagger: '2.0'
info:
  title: Test Service API
  description: >-
    Test API definition
  version: 1.0.0
host: api.test.swagger.codegen
schemes:
  - https
basePath: /testswagger
produces:
  - application/json
paths:
  '/namespaces/{namespace}':
    post:
      summary: Summary
      description: >-
                  Some description
      operationId: addNamespaces
      parameters:
        - name: namespace
          in: path
          description:  namespace containing
          required: true
          type: string
        - name: subnamespace
          in: body
          description: List of sub namespace
          required: false
          schema:
            type: array
            items:
              type: string
      tags:
        - namespaces
      responses:
        '200':
          description: Map of  namespace

        '404':
          description: Namespace not found

        '500':
          description: Internal server error

Command line used for generation

Steps to reproduce

Copy the yaml content into online Swagger Editor. Generate client using the cpprest
Try to compile. Compilation will fails at
"jsonArray.push_back( item.get() ? item->toJson() : web::json::value::null() );"

Related issues/PRs

Suggest a fix/enhancement

Reproduce the issue by importing the supplied YAML into Swagger Editor and generating a cpprest client. Start at the generated code around jsonArray.push_back( item.get() ? item->toJson() : web::json::value::null() );, then trace the cpprest generator output that produced it. Done means the generated client compiles for this array-of-strings request body.