Check that the value to Session.set is scalar (like we already do in Session.equals).
Check that the value to Session.set is scalar (like we already do in Session.equals).

Document this restriction for both functions.

We may later relax this (with deep-copy in and out of the Session); see issue

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor