Fix Flowable#toObservable backpressure support (#6321)

Author: philleonard

src/main/java/io/reactivex/Flowable.java

@@ -16983,7 +16983,7 @@ public final <K, V> Single<Map<K, Collection<V>>> toMultimap(
      * @since 2.0
      */
     @CheckReturnValue
-    @BackpressureSupport(BackpressureKind.NONE)
+    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Observable<T> toObservable() {
         return RxJavaPlugins.onAssembly(new ObservableFromPublisher<T>(this));