Default value for date and date-time fields has no effect

Description

An OpenAPI specification containing default values for date or date-time fields will have no effect on the generated code.

openapi-generator version

3.3.1

OpenAPI declaration file content or url

{
    "openapi": "3.0.0",
    "info": {"title":"blah", "version": "1.0.0"},
    "paths": {
        "/test1": {
            "post": {
                "tags": ["test"],
                "operationId": "testOp1",
                "responses": {
                    "200": {
                        "description": "successful operation"
                    }
                },
                "requestBody": {
                    "content": {
                        "application/json": {
                            "schema": {
                                "$ref": "#/components/schemas/Request1"
                            }
                        }
                    }
                }
            }
        }
    },
    "components": {
        "schemas": {
            "Request1": {
                "properties": {
                    "date_field": { 
                        "type":"string",
                        "format":"date",
                        "default":"2001-01-02"
                    },
                    "datetime_field": { 
                        "type":"string",
                        "format":"date-time",
                        "default":"2001-01-02T03:04:05.006"
                    }
                }
            }
        }
    }
}

Command line used for generation

It doesn't really matter what generator is used.

generate -v -g java -i foo.json -c rpdmcpp.config -o f:/temp/generated 

Steps to reproduce

Simple test:

• Run the generator command line
• Observe that generated Request1 date and dateTime fields are null:
  private LocalDate dateField = null;
  private OffsetDateTime datetimeField = null;

In-depth test:

• Run the generator command line in debugger.
• Break in Schema.setDefault()

public void setDefault(Object _default) {
    this._default = cast(_default);
}

• Note that _default has a valid string like "2001-01-02"
• Step into cast()
• It will fail to convert because !(String instanceof Date)

Related issues/PRs

None that I know of

Suggest a fix/enhancement

I don't know where to start. It seems that the bug is in a swagger library dependency not in openapi-generator itself. Perhaps there is a workaround to force a conversion?

Reproduce the Java generation command with the supplied OpenAPI declaration, then inspect Schema.setDefault() and cast(), where the report observes the string-to-date conversion failure. Trace how the generated Request1 date and dateTime fields receive defaults; done means they are initialized to the declared date and date-time values rather than null.