message: Add missing flatMap import
diff: import { Subject, of } from 'rxjs';
  -import { catchError, defaultIfEmpty, switchMap, tap } from 'rxjs/operators';
  +import { catchError, defaultIfEmpty, switchMap, flatMap, 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';
  