bug#28942: base64 decoding issue

[Aaron Schneider]

Using this website as reference:
http://www.xinxinbtw.com/lm/url_kd/f9aadd61ca33fd1d0c47296140e88faf1f3dd2a2.html


I try to decode the "thunder://" link to the "magnet://" link using base64.  
For me, it adds 'AA' at the beginning (0x4141) which shouldn't be there.

* Website:
------
thunder://QUFtYWduZXQ6P3h0PXVybjpidGloOmY5YWFkZDYxY2EzM2ZkMWQwYzQ3Mjk2MTQwZTg4ZmFmMWYzZGQyYTJaWg==
------
magnet:?xt=urn:btih:f9aadd61ca33fd1d0c47296140e88faf1f3dd2a2


* My base64 (happens on all versions I've tried):
echo 
QUFtYWduZXQ6P3h0PXVybjpidGloOmY5YWFkZDYxY2EzM2ZkMWQwYzQ3Mjk2MTQwZTg4ZmFmMWYzZGQyYTJaWg==
 | base64 -d
AAmagnet:?xt=urn:btih:f9aadd61ca33fd1d0c47296140e88faf1f3dd2a2ZZ

Which one is the right? Is this a bug or the website is removing the AA 
intentionally? Note that there are extensions for web browsers that 
automatically change the link like:
+ https://addons.mozilla.org/en-US/firefox/addon/ease-link/
+ https://chrome.google.com/webstore/detail/pfgjjlnidkopfimlhcfcjhakhifbnmof