Allow setting a connections uuid when in offline mode

proxy/src/main/java/net/md_5/bungee/connection/InitialHandler.java

@@ -571,6 +571,14 @@ public class InitialHandler extends PacketHandler implements PendingConnection
         this.onlineMode = onlineMode;
     }
 
+    @Override
+    public void setUniqueId(UUID uuid)
+    {
+        Preconditions.checkState( thisState == State.USERNAME, "Can only set uuid while state is username" );
+        Preconditions.checkState( !onlineMode, "Can only set uuid when online mode is false" );
+        this.uniqueId = uuid;
+    }
+
     @Override
     public String getUUID()
     {