[BUG] [typescript-fetch] Type mappings not applied to "helper functions"

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating the API definition with typescript-fetch and using "--type-mappings=Date=string", the types themselves get mapped correctly, but the helper functions are not adjusted accordingly.

I'd expect the helper functions to be updated to fit the new type.

export interface ApplicationState {
    * @type {string}
    * @memberof ApplicationState
    */
   enteredStateAt: string;
}

export function ApplicationStateFromJSONTyped(json: any, ignoreDiscriminator: boolean): ApplicationState {
   if ((json === undefined) || (json === null)) {
       return json;
   }
   return {
      // TYPESCRIPT COMPLAINS HERE
       'enteredStateAt': (new Date(json['enteredStateAt'])),
   };
}

export function ApplicationStateToJSON(value?: ApplicationState | null): any {
   if (value === undefined) {
       return undefined;
   }
   if (value === null) {
       return null;
   }
   return {
      // TYPESCRIPT COMPLAINS HERE
       'enteredStateAt': (value.enteredStateAt.toISOString()),
   };
}

openapi-generator version

"@openapitools/openapi-generator-cli": "^2.1.5",

Generation Details

openapi-generator generate -i [api.yml url] -g typescript-fetch -o ./src/.api-connector/ --skip-validate-spec --additional-properties modelPropertyNaming=original --type-mappings=Date=string

Steps to reproduce

1. Generate typescript-fetch API definition with type-mapping Date => string.
2. See Typescript gloriously complain.

Start by running the shown openapi-generator command with the typescript-fetch generator and --type-mappings=Date=string. Inspect the generated ApplicationStateFromJSONTyped and ApplicationStateToJSON helper functions alongside the mapped property type. Done means the generated helpers use the mapped string representation and the generated TypeScript no longer reports the shown type errors.