format: date results in uncompilable C++ Pistache code generated

Description

Swagger-codegen version

Swagger declaration file content or url

swagger: "2.0"
info:
  description: ...
  version: 1.1.1
  title: ...

basePath: /path
schemes:
- http

tags:
- name: api
  description: ...

paths:
  /somepath:
    get:
      tags:
      - Get
      summary: ...
      description: ...
      produces:
      - arrayOfobjectContainingString
      responses:
        200:
          description: Successful response
          schema:
            type: array
            items:
              $ref: "#/definitions/theresponse"

definitions:
  theresponse:
    type: object
    description: ...
    required:
      - type
    properties:
      date:
        type: string
        format: date
        description: ...
        example: some String

Steps to reproduce

Go to editor.swagger.io
Paste YAML above
Generate Pistache Server
Unzip the files, open model/Theresponse.cpp. on line 56 you will see the following:
newItem->fromJson(val["date"]);
which is trying to call "fromJson" on a std::string object. which will not compile.

This goes away if in the YAML file I remove "format: date". Not sure if it is directly related to that.

Related issues/PRs

Suggest a fix/enhancement

Reproduce the issue in editor.swagger.io with the provided YAML and generate the Pistache server. Inspect model/Theresponse.cpp around line 56 and trace the generated fromJson call for the date field; done means the generated C++ code compiles while retaining format: date.