[typescript-fetch] date=string type mapping

Created by: tristanlins

The typescript-fetch use Date typed properties for date and date-time fields. The generated interface and factory function looks like this:

export interface TheModel {
    /**
     * 
     * @type {Date}
     * @memberof TheModel
     */
    mandatoryDate: Date;
    /**
     * 
     * @type {Date}
     * @memberof TheModel
     */
    optionalDate?: Date;
}

export function TheModelFromJSONTyped(json: any, ignoreDiscriminator: boolean): TheModel {
    if ((json === undefined) || (json === null)) {
        return json;
    }
    return {
        'mandatoryDate': (new Date(json['updateDateTime'])),
        'optionalDate': !exists(json, 'publicationDateTime') ? undefined : (new Date(json['publicationDateTime'])),
    };
}

When changing the type mapping with --type-mappings=date=string,DateTime=string only the interface changes:

export interface TheModel {
    /**
     * 
     * @type {string}
     * @memberof TheModel
     */
    mandatoryDate: string;
    /**
     * 
     * @type {string}
     * @memberof TheModel
     */
    optionalDate?: string;
}

export function TheModelFromJSONTyped(json: any, ignoreDiscriminator: boolean): TheModel {
    if ((json === undefined) || (json === null)) {
        return json;
    }
    return {
        'mandatoryDate': (new Date(json['updateDateTime'])),
        'optionalDate': !exists(json, 'publicationDateTime') ? undefined : (new Date(json['publicationDateTime'])),
    };
}

But the factory function still converts all values to dates. This is wrong and results in a typescript compile error.

With the change, the generated code looks like this (when remap date=string,DateTime=string):

export interface TheModel {
    /**
     * 
     * @type {string}
     * @memberof TheModel
     */
    mandatoryDate: string;
    /**
     * 
     * @type {string}
     * @memberof TheModel
     */
    optionalDate?: string;
}

export function TheModelFromJSONTyped(json: any, ignoreDiscriminator: boolean): TheModel {
    if ((json === undefined) || (json === null)) {
        return json;
    }
    return {
        'mandatoryDate': json['updateDateTime'],
        'optionalDate': !exists(json, 'publicationDateTime') ? undefined : json['publicationDateTime'],
    };
}

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I just don't know how to cover that in the UnitTests. Which is why I didn't add any new tests :roll_eyes: It might make sense to set CodegenParameter#isDate & #isDateTime, CodegenProperty#isDate & #isDateTime and CodegenModel#isDate & #isDateTime to false instead. But that would be a more serious intervention.

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov

Relates to #11308 #10088