[QT5][QT5CPP] Bug in methods writing model data into JSON

Description

The generated model classes contain methods to read data from and write data back to JSON. I'm expecting that I'd receive the same JSON object when:

auto model = new SWGMyModel();
auto jsonOutput = model->fromJson()->asJson();

Instead on the output most of the fields are not defined anymore.

The reason for that is that the m_*_isSet member variables do not get updated when loading data from a JSON string / object

Swagger-codegen version

Version 2.4.14

Swagger declaration file content or url

swagger: "2.0"
info:
  version: "1.0.0"
  title: "Bug in qt5cpp"
schemes:
- "http"
paths:

  /echo:
    post:
      parameters:
      - name: "body"
        in: "body"
        schema:
          $ref: "#/definitions/Example"
      responses:
        "200":
          description: "successful operation"

definitions:
  Example:
    type: "object"
    properties:
      int64_type:
        type: "integer"
        format: "int64"
      int32_type:
        type: "integer"
        format: "int32"
      boolean_type:
        type: "boolean"
      float_type:
        type: "number"
        format: "float"
      double_type:
        type: "number"
        format: "double"
      string_type:
        type: "string"
      enum_type:
        type: "string"
        enum:
        - "a"
        - "b"
      object_type:
        type: "object"
        properties:
          double_type:
            type: "number"
            format: "double"
          string_type:
            type: "string"
      number_array_type:
        type: "array"
        items:
          type: "string"
      string_array_type:
        type: "array"
        items:
          type: "string"

Command line used for generation

java -jar swagger-codegen-cli.jar generate -l qt5cpp -i swagger.json

Steps to reproduce

1. generate code using provided interface description
2. use these files to create a Qt project using the generated client
3. run qmake && make
4. check the output and compare to the JSON used as input

Related issues/PRs

Suggest a fix/enhancement

I don't know the purpose of m_*_isSet member variables. Either they are used to track if the user modified the model (like the use in the isSet() function suggests). Then the check for these members when writing to JSON can be removed.
Or the members are to check if the value is set at all (not undefined) then we could set the member to true if the corresponding field in the JSON document is present and add lines like the following in the fromJsonObject() method:

    m_propertyName_isSet = pJson.contains("propertyName");

Start with the qt5cpp generator templates that produce model fromJsonObject() and asJson(), then compare how m_*_isSet is handled during deserialization and serialization. Reproduce the issue with the supplied Swagger declaration and generation command; it is done when fields present in the input JSON are preserved in the generated model's output.