Support blocking with: scalaFuture.toJava.toCompletableFuture

I believe this implements the somewhat reluctant consensus
reached in scala#43. Go with the flow by allowing people to block
using the standard Java 8 idiom of `toCompletableFuture.get`.

I've added a comment and a test to caution that calling
`toCompletableFuture.complete(value)` does *not* effect the underlying
Promise.

Fixes scala#43

Author: retronym

src/main/scala/scala/concurrent/java8/FutureConvertersImpl.scala

@@ -66,8 +66,15 @@ object FuturesConvertersImpl {
       cf
     }
 
-    override def toCompletableFuture(): CompletableFuture[T] =
-      throw new UnsupportedOperationException("this CompletionStage represents a read-only Scala Future")
+    /**
+     * @inheritdoc
+     *
+     * WARNING: completing the result of this method will not complete the underlying
+     *          Scala Future or Promise.
+     */
+    override def toCompletableFuture(): CompletableFuture[T] = {
+      this // TODO or maybe `thenApply(JF.identity())`
+    }
 
     override def toString: String = super[CompletableFuture].toString
   }

src/test/java/scala/compat/java8/FutureConvertersTest.java

@@ -318,4 +318,27 @@ public void testToJavaExceptionally() throws InterruptedException,
         latch.countDown();
         assertEquals("Hello", second.toCompletableFuture().get());
     }
+
+    @Test
+    public void testToJavaToCompletableFuture() throws ExecutionException, InterruptedException {
+        final Promise<String> p = promise();
+        final CompletionStage<String> cs = toJava(p.future());
+        CompletableFuture<String> cf = cs.toCompletableFuture();
+        assertEquals("notyet", cf.getNow("notyet"));
+        p.success("done");
+        assertEquals("done", cf.get());
+    }
+
+    @Test
+    public void testToJavaToCompletableFutureDoesNotMutateUnderlyingPromise() throws ExecutionException, InterruptedException {
+        final Promise<String> p = promise();
+        Future<String> sf = p.future();
+        final CompletionStage<String> cs = toJava(sf);
+        CompletableFuture<String> cf = cs.toCompletableFuture();
+        assertEquals("notyet", cf.getNow("notyet"));
+        cf.complete("done");
+        assertEquals("done", cf.get());
+        assertFalse(sf.isCompleted());
+        assertFalse(p.isCompleted());
+    }
 }