[typescript-fetch] (4.0.X) unioned types incorrectly deserialized

Description

The generated deserializer for unioned types incorrectly inlines the unioned types

openapi-generator version

openapi-generator-cli-4.0.0-20181126.024631-44

OpenAPI declaration file content or url

swagger: "2.0"
info:
  title: "Simple Test"

paths:
  "/get/Simple":
    get:
      summary: Retrieve List of Simple
      operationId: "get.Simple"
      responses:
        200:
          description: OK
          schema:
            allOf:
              - type: object
                properties:
                  items:
                    type: array
                    items:
                      $ref: "#/definitions/Simple"
              - $ref: "#/definitions/Paging"
definitions:         
  Simple:
    type: object
    required:
      - Name
    properties:
      Date:
        type: string
        format: date
  Paging:
    properties:
      more:
        type: boolean
      cursor:
        type: string

Generated Code (excerpt):

export interface InlineResponse200 {
    /**
     * 
     * @type {Paging}
     * @memberof InlineResponse200
     */
    paging?: Paging;
    /**
     * 
     * @type {Array<Simple>}
     * @memberof InlineResponse200
     */
    items?: Array<Simple>;
}

export function InlineResponse200FromJSON(json: any): InlineResponse200 {
    return {
        'paging': !exists(json, 'Paging') ? undefined : PagingFromJSON(json['Paging']),
        'items': !exists(json, 'items') ? undefined : (json['items'] as Array<any>).map(SimpleFromJSON),
    };
}

Example response (notice no "Paging" key):

{
  "items": [
    {
      "Date": "2018-11-27"
    }
  ],
  "more": true,
  "cursor": "string"
}

Command line used for generation

openapi-generator generate -i openapi.yaml -l typescript-fetch -c es6.json -o ./typescript-api/

Steps to reproduce

1. Use the provided yaml excerpt above to generate a client with the provided command line
2. Buggy code generated

Related issues/PRs

PR #569

Suggest a fix/enhancement

Not sure - the debugModels output makes it look like the generated code is accurate, there is something automatically creating a nested key for the composed model which should have its properties extend the model as it is not named.

Any ideas on how to tackle this? It almost appears as if the models passed to the template is incorrect (adding a named var Paging instead of its properties).

cc @Place1

Start by running the provided OpenAPI YAML with the stated openapi-generator command and inspect the generated typescript-fetch InlineResponse200FromJSON function. Compare its Paging lookup with the flattened example response, then use the linked debugModels output and PR #569 to trace where the composed model becomes a nested named property. Done means unioned or composed properties deserialize from the response shape without requiring a nonexistent Paging key.