openapi2conv: convert references in nested additionalProperties schemas

[travisnewhouse]

When the schema specified in additionalProperties contains another nested schema with additionalProperties, the schema references must be converted at all of levels of the nested schemas.

The following examples illustrate the problem. This pull request fixes the issue by traversing all of the nested schemas in additionalProperties, making copies of the objects that are modified by the conversion.

Prior to this change, a schema reference is specified in a top-level additionalProperties was being converted correctly. In the following example OpenAPI 2.0 definition, the reference to "#/definitions/Foo" is correctly converted to "#/components/schemas/Foo" in the OpenAPI 3.0 definition that is produced.

{
    "basePath": "/v2",
    "host": "test.example.com",
    "info": {
        "title": "MyAPI",
        "version": "0.1"
    },
    "paths": {
        "/foo": {
            "get": {
                "operationId": "getFoo",
                "produces": [
                    "application/json"
                ],
                "responses": {
                    "200": {
                        "description": "returns all information",
                        "schema":{
                            "type":"object",
                            "additionalProperties":{
                               "$ref":"#/definitions/Foo"
                            }
                        }
                    }
                },
                "summary": "get foo"
            }
        }
    },
        "definitions": {
            "Foo": {
                    "type": "object",
                        "properties": {
                            "a": {
                                    "type": "string"
                                }
                        }
                }
        },
    "schemes": [
        "http"
    ],
    "swagger": "2.0"
}

When there is a schema reference in an additionalProperties that is nested in the schema of another additionalProperties, then the conversion from OpenAPI 2.0 to OpenAPI 3.0 did not convert the schema reference to "#/components/schema/Foo". This case is fixed by this pull request.

{
    "basePath": "/v2",
    "host": "test.example.com",
    "info": {
        "title": "MyAPI",
        "version": "0.1"
    },
    "paths": {
        "/foo": {
            "get": {
                "operationId": "getFoo",
                "produces": [
                    "application/json"
                ],
                "responses": {
                    "200": {
                        "description": "returns all information",
                        "schema":{
                            "type":"object",
                            "additionalProperties":{
                               "type":"object",
                               "additionalProperties":{
                                   "$ref":"#/definitions/Foo"
                               }
                            }
                        }
                    }
                },
                "summary": "get foo"
            }
        }
    },
        "definitions": {
            "Foo": {
                    "type": "object",
                        "properties": {
                            "a": {
                                    "type": "string"
                                }
                        }
                }
        },
    "schemes": [
        "http"
    ],
    "swagger": "2.0"
}