[PHP] additionalProperties in referenced model ignored

Description

If I use a reference in a response object for a property, additionalProperties are ignored.

It works fine, when I include it directly.

When I explicitly define some additional properties, those are extracted as well (in both approaches), just the additional Properties are not

Swagger-codegen version

2.3.1

Swagger declaration file content or url

Not working as expected

swagger: '2.0'
info:
  title: test
  version: 1.0.0
paths:
  /response.json:
    get:
      operationId: listOperations
      responses:
        '200':
          description: just a test
          schema:
            type: object
            additionalProperties:
              $ref: "#/definitions/some_definition"
definitions:
  some_definition:
    description: Some values
    additionalProperties: {}
    type: object

Working as expected:

swagger: '2.0'
info:
  title: test
  version: 1.0.0
paths:
  /response.json:
    get:
      operationId: listOperations
      responses:
        '200':
          description: just a test
          schema:
            type: object
            additionalProperties:
              description: Some values
              additionalProperties: {}
              type: object

Response JSON:

{
    "hello": {
      "world": "and all",
      "description": "a bar test"
    },
    "lala": {
      "anotherone": "and all"
    }
}

Command line used for generation

`swagger-codegen generate -i http://localhost/swagger.yaml -l php

Steps to reproduce

<?php
require_once(__DIR__ . '/vendor/autoload.php');
$apiInstance = new Swagger\Client\Api\DefaultApi(
// If you want use custom http client, pass your client which implements `GuzzleHttp\ClientInterface`.
// This is optional, `GuzzleHttp\Client` will be used as default.
    new GuzzleHttp\Client(),
);

    $result = $apiInstance->listOperations();
    print_r($result);

Should return an Array with all values (or a model with all values), does return Swagger\Client\Model\SomeDefinition with just the explicitly defined properties (if such are defined)

Suggest a fix/enhancement

I don't know where/why that happened and if it's intended behaviour or not. If you need more information, I'm happy to provide those.

Start by running the supplied Swagger YAML through swagger-codegen generate with the php language and inspect the generated SomeDefinition response model. Trace how additionalProperties is handled when its schema is a $ref versus an inline object. Done means the generated PHP client preserves arbitrary properties from the referenced model, not only explicitly defined ones.