Fixing PubRelEncoder to encode PubRel message flag to match what is in the MQTT specification http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718054

netty_parser/src/main/java/org/eclipse/moquette/parser/netty/PubRelEncoder.java

@@ -28,7 +28,7 @@ class PubRelEncoder extends DemuxEncoder<PubRelMessage> {
 
     @Override
     protected void encode(ChannelHandlerContext chc, PubRelMessage msg, ByteBuf out) {
-        out.writeByte(AbstractMessage.PUBREL << 4);
+        out.writeByte(AbstractMessage.PUBREL << 4 | 0x02);
         out.writeBytes(Utils.encodeRemainingLength(2));
         out.writeShort(msg.getMessageID());
     }