[Javascript] Undeclared properties in the Object Model

Description

Properties not declared in Object type are ignored.
However, unless additionalProperties is specified, it should be incorporated.

definitions:
  # It's a free form object accept any key/value.
  myObject:
    type: object

Generated Code snippet:
obj is always empty object {}.

  exports.constructFromObject = function(data, obj) {
    if (data) {
      obj = obj || new exports();

    }
    return obj;
  }

Swagger-codegen version

c91ce17aeeeae2982b4a648333835377adc8d054

Swagger declaration file content or url

swagger: '2.0'
info:
  title: Test yaml 
  version: '1.0.0'
basePath: /api
schemes:
  - https
  - http
consumes:
  - application/json
produces:
  - application/json

paths:

  '/books/{id}':
    parameters:
      - BookID:
        name: id
        in: path
        description: Book ID
        required: true
        type: integer
        format: int64
    get:
      operationID: GetBook
      description: |
        Get book
      responses:
        '200':
          description: Succeeded.
          schema:
            $ref: '#/definitions/Book' 
        '404':
          description: Not found.
          schema:
            type: object
    put:
      operationID: PutBook
      description: |
        Put book
      parameters:
        - name: PutBook
          description: Book data.
          in: body
          required: true
          schema:
            $ref: '#/definitions/Book' 
      responses:
        '200':
          description: Succeeded.
          schema:
            type: object

definitions:

  Book:
    type: object

Command line used for generation

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate  -i test.yml  -l javascript  -o js

Related issues/PRs

Similar to #4973

Suggest a fix/enhancement

If additionalProperties is not supplied, copy props in data to obj

Start with the JavaScript generator entry point that emits the shown constructFromObject function, then reproduce generation using the supplied Swagger YAML and command. Check the generated object for undeclared properties when additionalProperties is omitted. Done means free-form object data is retained while the existing behavior for declared properties remains intact.