@Override
    public Observable<O> applyAsync(I input) {
        return Observable.just(this.apply(input));
    }
A wrapper implementation of applyAsync() that is intended just to aggregate a non-blocking apply() method
in an Observable.

A subclass filter should override this method if doing any IO.
zuul-core/src/main/java/com/netflix/zuul/filters/BaseSyncFilter.java