title: Cannot resolve method 'subscribe(anonymous io.reactivex.functionx.Consumer<java.util.List<...>>)' in rxjava2?
body: <p>after toList operator performs, original Flowable&lt;&lt;\List>> converts to Single&lt;&lt;\List>>. and it turns out if I create Consumer to subscribe to Single,
  the Consumer value type cannot be changed except Object? </p>
  
  <pre><code> @Override
  public void loadBannerData(final ADFilterType adFilterType) {
      remoteListDataSource.getBannerListData(adFilterType)
              .flatMap(new Function&lt;List&lt;BannerBeanList.BannerBean&gt;, Publisher&lt;?&gt;&gt;() {
                  @Override
                  public Publisher&lt;?&gt; apply(List&lt;BannerBeanList.BannerBean&gt; bannerBeen) throws Exception {
                      return Flowable.fromIterable(bannerBeen);
                  }
              })
              .toList()
              .subscribeOn(Schedulers.io())
              .observeOn(AndroidSchedulers.mainThread())
             /******************************Consume Value Type**************************
              .subscribe(new Consumer&lt;List&lt;BannerBeanList.BannerBean&gt;&gt;() {
                  @Override
                  public void accept(List&lt;BannerBeanList.BannerBean&gt; bannerBeens) throws Exception {
                      mainTabView.showMainBanner(bannerBeens);
                  }
              });
            *****************************************************************************/
  }
  </code></pre>