@Override
    public void onUnknownFrame(ChannelHandlerContext ctx, byte frameType, int streamId,
                               Http2Flags flags, ByteBuf payload) {}
Sends an {@link Http2Headers} with the specified {@link HttpResponseStatus} and
{@code DATA} with the specified {@code payload}.
core/src/test/java/com/linecorp/armeria/client/SimpleH2CServerHandler.java