Use Action instead of Func0<Void>

As per ReactiveX#1899 (comment)

Author: benjchristensen

src/main/java/rx/internal/operators/OperatorOnBackpressureBuffer.java

@@ -24,14 +24,14 @@
 import rx.Observable.Operator;
 import rx.Producer;
 import rx.Subscriber;
-import rx.functions.Func0;
+import rx.functions.Action0;
 
 public class OperatorOnBackpressureBuffer<T> implements Operator<T, T> {
 
     private final NotificationLite<T> on = NotificationLite.instance();
 
     private final Long capacity;
-    private final Func0 onOverflow;
+    private final Action0 onOverflow;
 
     public OperatorOnBackpressureBuffer() {
         this.capacity = null;
@@ -42,7 +42,7 @@ public OperatorOnBackpressureBuffer(long capacity) {
         this(capacity, null);
     }
 
-    public OperatorOnBackpressureBuffer(long capacity, Func0<Void> onOverflow) {
+    public OperatorOnBackpressureBuffer(long capacity, Action0 onOverflow) {
         if (capacity <= 0) {
             throw new IllegalArgumentException("Buffer capacity must be > 0");
         }

src/test/java/rx/internal/operators/OperatorOnBackpressureBufferTest.java

@@ -15,6 +15,9 @@
  */
 package rx.internal.operators;
 
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
 import java.nio.BufferOverflowException;
 import java.util.concurrent.CountDownLatch;
 
@@ -25,14 +28,11 @@
 import rx.Observer;
 import rx.Subscriber;
 import rx.Subscription;
-import rx.functions.Func0;
+import rx.functions.Action0;
 import rx.observables.ConnectableObservable;
 import rx.observers.TestSubscriber;
 import rx.schedulers.Schedulers;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertTrue;
-
 public class OperatorOnBackpressureBufferTest {
 
     @Test
@@ -126,11 +126,10 @@ public void onNext(Long t) {
                     .publish();
         final ConnectableObservable<Long> batch =
             infinite.subscribeOn(Schedulers.computation())
-                    .onBackpressureBuffer(100, new Func0<Void>() {
+                    .onBackpressureBuffer(100, new Action0() {
                         @Override
-                        public Void call() {
+                        public void call() {
                             l3.countDown();
-                            return null;
                         }
                     }).publish();
         Subscription s = batch.subscribe(ts);