fix: added missing $is_beta class property to FS_User

[bfintal]

Fixes #675

[swashata]

Hey @bfintal thank you for the PR. As I can see it is adding a property to the FS_User class. But we have moved the is_beta from user to site/install quite some time back.

So could it be that the error you're getting is due to some old/legacy account data? We have also incorporated a garbage collector, so that old data might have gotten deleted already.

Could you please confirm if this fix is still needed, even after running the garbage collector?

[andreiglingeanu]

@swashata I can confirm that removing the cached serialized objects from the DB helped fix this issue in my end.
So yes, indeed adding this flag in the class is not needed from my point of view.