Fix voidPromise in Http2FrameCodec.writeHeadersFrame (#13958)

Motivation:

Writing a Http2HeadersFrame on a new stream with a void promise would
throw an IllegalStateException.

Modification:

Unvoid the promise in the code that requires adding a listener.

Result:

No IllegalStateException.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java

@@ -403,12 +403,14 @@ private void writeGoAwayFrame(ChannelHandlerContext ctx, Http2GoAwayFrame frame,
     }
 
     private void writeHeadersFrame(final ChannelHandlerContext ctx, Http2HeadersFrame headersFrame,
-                                   final ChannelPromise promise) {
+                                   ChannelPromise promise) {
 
         if (isStreamIdValid(headersFrame.stream().id())) {
             encoder().writeHeaders(ctx, headersFrame.stream().id(), headersFrame.headers(), headersFrame.padding(),
                     headersFrame.isEndStream(), promise);
         } else if (initializeNewStream(ctx, (DefaultHttp2FrameStream) headersFrame.stream(), promise)) {
+            promise = promise.unvoid();
+
             final int streamId = headersFrame.stream().id();
 
             encoder().writeHeaders(ctx, streamId, headersFrame.headers(), headersFrame.padding(),

codec-http2/src/test/java/io/netty/handler/codec/http2/Http2FrameCodecTest.java

@@ -844,6 +844,13 @@ public void operationComplete(ChannelFuture future) throws Exception {
         assertTrue(listenerExecuted.get());
     }
 
+    @Test
+    public void writeHeadersVoidPromise() {
+        final Http2FrameStream stream = frameCodec.newStream();
+        channel.writeAndFlush(new DefaultHttp2HeadersFrame(new DefaultHttp2Headers()).stream(stream),
+                channel.voidPromise());
+    }
+
     @Test
     public void upgradeEventNoRefCntError() throws Exception {
         frameInboundWriter.writeInboundHeaders(Http2CodecUtil.HTTP_UPGRADE_STREAM_ID, request, 31, false);