message: fix(core): use `mergeMap` instead of `flatMap`
diff: import { Subject, of } from 'rxjs';
  -import { catchError, defaultIfEmpty, switchMap, flatMap, tap } from 'rxjs/operators';
  +import { catchError, defaultIfEmpty, switchMap, mergeMap, tap } from 'rxjs/operators';
  import { combineEffects, combineMiddlewareEffects } from './effects/effects.combiner';
  import { Effect, EffectResponse, GroupedEffects } from './effects/effects.interface';
  import { Http, HttpRequest, HttpResponse, HttpStatus } from './http.interface';
  @@ -16,7 +16,7 @@ export const httpListener = ({ middlewares = [], effects, errorMiddleware }: Htt
  const request$ = new Subject<Http>();
  const effect$ = request$
  .pipe(
  - flatMap(({ req, res }) =>
  + mergeMap(({ req, res }) =>
  combineMiddlewareEffects(middlewares)(res)(req)
  .pipe(
  switchMap(combineEffects(effects)(res)),
  