Consider to allow Request interface to be used in endpoint

Now endpoint is only allowing `string` or `function`. Do you think it makes sense to allow [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request) interface to be whitelisted as well? Since the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/GlobalFetch/fetch) is accepting both `string` and `Request` object.

https://github.com/agraboso/redux-api-middleware/blob/master/src/validation.js#L107