AbstractFuture should not wrap CancellationException

Motivation:
AbstractFuture currently wraps CancellationException in a ExecutionException. However the interface of Future says that this exception should be directly thrown.

Modifications:
- Throw CancellationException from AbstractFuture.get

Result:
Interface contract for CancellationException is honored in AbstractFuture.

common/src/main/java/io/netty/util/concurrent/AbstractFuture.java

@@ -15,6 +15,7 @@
  */
 package io.netty.util.concurrent;
 
+import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
@@ -34,6 +35,9 @@ public abstract class AbstractFuture<V> implements Future<V> {
         if (cause == null) {
             return getNow();
         }
+        if (cause instanceof CancellationException) {
+            throw (CancellationException) cause;
+        }
         throw new ExecutionException(cause);
     }
 
@@ -44,6 +48,9 @@ public abstract class AbstractFuture<V> implements Future<V> {
             if (cause == null) {
                 return getNow();
             }
+            if (cause instanceof CancellationException) {
+                throw (CancellationException) cause;
+            }
             throw new ExecutionException(cause);
         }
         throw new TimeoutException();

common/src/test/java/io/netty/util/concurrent/DefaultPromiseTest.java

@@ -19,9 +19,12 @@ package io.netty.util.concurrent;
 import org.junit.Test;
 
 import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CancellationException;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import static org.hamcrest.CoreMatchers.is;
@@ -33,6 +36,21 @@ import static org.junit.Assert.assertTrue;
 @SuppressWarnings("unchecked")
 public class DefaultPromiseTest {
 
+    @Test(expected = CancellationException.class)
+    public void testCancellationExceptionIsThrownWhenBlockingGet() throws InterruptedException, ExecutionException {
+        final Promise<Void> promise = new DefaultPromise<Void>(ImmediateEventExecutor.INSTANCE);
+        promise.cancel(false);
+        promise.get();
+    }
+
+    @Test(expected = CancellationException.class)
+    public void testCancellationExceptionIsThrownWhenBlockingGetWithTimeout() throws InterruptedException,
+            ExecutionException, TimeoutException {
+        final Promise<Void> promise = new DefaultPromise<Void>(ImmediateEventExecutor.INSTANCE);
+        promise.cancel(false);
+        promise.get(1, TimeUnit.SECONDS);
+    }
+
     @Test
     public void testNoStackOverflowErrorWithImmediateEventExecutorA() throws Exception {
         final Promise<Void>[] p = new DefaultPromise[128];