that should be working, I just tried it against our internal mail which is running M2. If the account you are going to isn't the default domain name, then make sure you use /service/user/user@domain/calendar.ics, or better yet just use: /service/user/~/calendar.ics.
Note, you also have to type in
user@domain.name as the user name in the basic auth dialog box if you don't have the default domain set.
roland