Fix SelectableChannel support in NioEventLoop (#8344)

Motivation:

Unless the 'io.netty.noKeySetOptimization' system property is set,
registering a SelectableChannel instance to a NioEventLoop results
in a ClassCastException:

    io.netty.channel.nio.SelectedSelectionKeySetSelector cannot be cast
        to java.nio.channels.spi.AbstractSelector

Modifications:

Instead of 'selector', pass 'unwrappedSelector' to SelectableChannel.

Result:

It is possible to register a SelectableChannel instance without
setting the 'io.netty.noKeySetOptimization' system property.

transport/src/main/java/io/netty/channel/nio/NioEventLoop.java

@@ -294,7 +294,7 @@ public final class NioEventLoop extends SingleThreadEventLoop {
         }
 
         try {
-            ch.register(selector, interestOps, task);
+            ch.register(unwrappedSelector, interestOps, task);
         } catch (Exception e) {
             throw new EventLoopException("failed to register a channel", e);
         }

transport/src/test/java/io/netty/channel/nio/NioEventLoopTest.java

@@ -24,7 +24,10 @@ import io.netty.channel.socket.nio.NioServerSocketChannel;
 import io.netty.util.concurrent.Future;
 import org.junit.Test;
 
+import java.net.InetSocketAddress;
+import java.nio.channels.SelectionKey;
 import java.nio.channels.Selector;
+import java.nio.channels.SocketChannel;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
@@ -132,4 +135,40 @@ public class NioEventLoopTest extends AbstractEventLoopTest {
             group.shutdownGracefully();
         }
     }
+
+    @Test(timeout = 1000)
+    public void testSelectableChannel() throws Exception {
+        NioEventLoopGroup group = new NioEventLoopGroup(1);
+        NioEventLoop loop = (NioEventLoop) group.next();
+
+        try {
+            Channel channel = new NioServerSocketChannel();
+            loop.register(channel).syncUninterruptibly();
+            channel.bind(new InetSocketAddress(0)).syncUninterruptibly();
+
+            SocketChannel selectableChannel = SocketChannel.open();
+            selectableChannel.configureBlocking(false);
+            selectableChannel.connect(channel.localAddress());
+
+            final CountDownLatch latch = new CountDownLatch(1);
+
+            loop.register(selectableChannel, SelectionKey.OP_CONNECT, new NioTask<SocketChannel>() {
+                @Override
+                public void channelReady(SocketChannel ch, SelectionKey key) {
+                    latch.countDown();
+                }
+
+                @Override
+                public void channelUnregistered(SocketChannel ch, Throwable cause) {
+                }
+            });
+
+            latch.await();
+
+            selectableChannel.close();
+            channel.close().syncUninterruptibly();
+        } finally {
+            group.shutdownGracefully();
+        }
+    }
 }