Do a better job of passing channel (un)registration to the client.

Also fixes a regression related to WDLCompanion.

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -95,8 +95,12 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     // the client.
     if (PluginMessageUtil.isRegister(packet)) {
       serverConn.getPlayer().getKnownChannels().addAll(PluginMessageUtil.getChannels(packet));
+      serverConn.getPlayer().getMinecraftConnection().write(packet);
+      return true;
     } else if (PluginMessageUtil.isUnregister(packet)) {
       serverConn.getPlayer().getKnownChannels().removeAll(PluginMessageUtil.getChannels(packet));
+      serverConn.getPlayer().getMinecraftConnection().write(packet);
+      return true;
     }
 
     if (PluginMessageUtil.isMcBrand(packet)) {

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/PluginMessage.java

@@ -47,7 +47,7 @@ public class PluginMessage implements MinecraftPacket {
   @Override
   public void decode(ByteBuf buf, ProtocolUtils.Direction direction, ProtocolVersion version) {
     this.channel = ProtocolUtils.readString(buf);
-    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13) < 0) {
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13) >= 0) {
       this.channel = transformLegacyToModernChannel(this.channel);
     }
     this.data = new byte[buf.readableBytes()];
@@ -59,8 +59,11 @@ public class PluginMessage implements MinecraftPacket {
     if (channel == null) {
       throw new IllegalStateException("Channel is not specified.");
     }
-    ProtocolUtils.writeString(buf, version.compareTo(ProtocolVersion.MINECRAFT_1_13) >= 0
-        ? channel : transformLegacyToModernChannel(this.channel));
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_13) >= 0) {
+      ProtocolUtils.writeString(buf, transformLegacyToModernChannel(this.channel));
+    } else {
+      ProtocolUtils.writeString(buf, this.channel);
+    }
     buf.writeBytes(data);
   }