message: style(example): iif usage in auth.middleware
diff: import { Effect, HttpError, HttpRequest, HttpStatus } from '@marblejs/core';
  -import { Observable, merge, throwError } from 'rxjs';
  -import { filter, switchMap } from 'rxjs/operators';
  +import { iif, of, throwError } from 'rxjs';
  +import { switchMap } from 'rxjs/operators';
  import { isAuthorized } from '../fakes/auth.fake';
  -const authorized$ = (req$: Observable<HttpRequest>) =>
  - req$.pipe(
  - filter(req => isAuthorized(req)),
  - );
  -
  -const unauthorized$ = (req$: Observable<HttpRequest>) =>
  - req$.pipe(
  - filter(req => !isAuthorized(req)),
  - switchMap(req =>
  - throwError(new HttpError('Unauthorized', HttpStatus.UNAUTHORIZED)),
  - ),
  - );
  -
  export const authorize$: Effect<HttpRequest> = request$ =>
  request$.pipe(
  - switchMap(() => merge(
  - authorized$(request$),
  - unauthorized$(request$),
  + switchMap(req => iif(
  + () => !isAuthorized(req),
  + throwError(new HttpError('Unauthorized', HttpStatus.UNAUTHORIZED)),
  + of(req),
  )),
  );
  