2.x: Improve the Observable/Flowable cache() operators (ReactiveX#6275)

* 2.x: Improve the Observable/Flowable cache() operators

* Remove unnecessary casting.

* Remove another unnecessary cast.

Author: akarnokd

src/main/java/io/reactivex/Observable.java

@@ -6164,7 +6164,7 @@ public final <B, U extends Collection<? super T>> Observable<U> buffer(Callable<
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Observable<T> cache() {
-        return ObservableCache.from(this);
+        return cacheWithInitialCapacity(16);
     }
 
     /**
@@ -6222,7 +6222,8 @@ public final Observable<T> cache() {
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Observable<T> cacheWithInitialCapacity(int initialCapacity) {
-        return ObservableCache.from(this, initialCapacity);
+        ObjectHelper.verifyPositive(initialCapacity, "initialCapacity");
+        return RxJavaPlugins.onAssembly(new ObservableCache<T>(this, initialCapacity));
     }
 
     /**