Pass subclass specific properties to template as vars

[giavjeko]

Models with class inheritance are not rendered correctly in any language. When rendering a template, DefaultCodegenConfig does not pass subclass properties as vars param.

For example, if there are my models (JSON OpenAPI specification v3):

{
  "schemas": {
    "BaseClass": {
      "type": "object",
      "properties": {
        "baseProperty": {
          "type": "string"
        }
      }
    },
    "SubClass": {
      "type": "object",
      "properties": {
        "subProperty": {
          "type": "string"
        }
      },
      "allOf": [
        {
          "$ref": "#/components/schemas/BaseClass"
        }
      ]
    }
  }
}

then SubClass will be rendered without subProperty.

This change fixes the issue and renders subclasses correctly.