Use configured ByteBufAllocator in InboundHttp2ToHttpAdapter (#9611)

Motivation:

At the moment we use Unpooled.buffer(...) in InboundHttp2ToHttpAdapter when we need to do a copy of the message. We should better use the configured ByteBufAllocator for the Channel

Modifications:

Change internal interface to also take the ByteBufAllocator as argument and use it when we need to allocate a ByteBuf.

Result:

Use the "correct" ByteBufAllocator in InboundHttp2ToHttpAdapter in all cases

codec-http2/src/main/java/io/netty/handler/codec/http2/InboundHttp2ToHttpAdapter.java

@@ -16,7 +16,6 @@
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
-import io.netty.buffer.Unpooled;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.http.FullHttpMessage;
 import io.netty.handler.codec.http.FullHttpRequest;
@@ -53,9 +52,9 @@ public boolean mustSendImmediately(FullHttpMessage msg) {
         }
 
         @Override
-        public FullHttpMessage copyIfNeeded(FullHttpMessage msg) {
+        public FullHttpMessage copyIfNeeded(ByteBufAllocator allocator, FullHttpMessage msg) {
             if (msg instanceof FullHttpRequest) {
-                FullHttpRequest copy = ((FullHttpRequest) msg).replace(Unpooled.buffer(0));
+                FullHttpRequest copy = ((FullHttpRequest) msg).replace(allocator.buffer(0));
                 copy.headers().remove(HttpHeaderNames.EXPECT);
                 return copy;
             }
@@ -200,7 +199,7 @@ protected FullHttpMessage processHeadersBegin(ChannelHandlerContext ctx, Http2St
         if (sendDetector.mustSendImmediately(msg)) {
             // Copy the message (if necessary) before sending. The content is not expected to be copied (or used) in
             // this operation but just in case it is used do the copy before sending and the resource may be released
-            final FullHttpMessage copy = endOfStream ? null : sendDetector.copyIfNeeded(msg);
+            final FullHttpMessage copy = endOfStream ? null : sendDetector.copyIfNeeded(ctx.alloc(), msg);
             fireChannelRead(ctx, msg, release, stream);
             return copy;
         }
@@ -354,9 +353,10 @@ private interface ImmediateSendDetector {
          * with a 'Expect: 100-continue' header. The message will be sent immediately,
          * and the data will be queued and sent at the end of the stream.
          *
+         * @param allocator The {@link ByteBufAllocator} that can be used to allocate
          * @param msg The message which has just been sent due to {@link #mustSendImmediately(FullHttpMessage)}
          * @return A modified copy of the {@code msg} or {@code null} if a copy is not needed.
          */
-        FullHttpMessage copyIfNeeded(FullHttpMessage msg);
+        FullHttpMessage copyIfNeeded(ByteBufAllocator allocator, FullHttpMessage msg);
     }
 }