Chargers typically aren't grounded. The AC and output sides of the charger are electrically isolated, but it seems like that isolation isn't working in your case. It's not uncommon for enough high-frequency current to leak through to be felt, but based on the description it sounds like you've got full AC leaking through. I strongly recommend you don't use this charger, since there's a real risk of electrocution in that case. I wrote the original article, so if you send me this charger I'll tear it down, figure out what's wrong, and post a writeup (my email is on the article page).