fixes #814

Bleh, this was a dumb error.  When you say ``session.merge(attendee)`` it doesn't actually merge ``attendee`` into the session, it returns a new object.  In all of the other places where we use ``session.merge`` we (correctly) do the merge after the changes were made to the object, but in this function and only this function, I accidentally did the merge beforehand and then neglected to assign the return value to the variable being mutated.