The problem here is that when you are constructing a message the
boundary is calculated "as needed", and returning the list of items is
not one of the times when the boundary is needed.  I agree that it is
not good design that as_string can mutate the object it is called on;
however the alternative (throwing away the calculated boundary after
message Generation) means that stringifying the same message twice would
produce two strings that are not equal...which also seems like bad
design.  But perhaps that would be less bad design.

The underlying problem here is that the things being returned by items
are strings, when they should be Header objects.  That problem isn't
going to be resolved until the email package rewrite happens.