Right, the join behavior is correct.  But the definition of a *default* (aka sentinel) value is that it is returned *unchanged*.  It is counter intuitive that if you pass a list of strings as a default, that what you get back is those strings joined by ','.  But that is the released behavior. so we have to live with it.

I think the doc patch is OK except that it should be 'iterable' rather than 'iterator'.  However, it might make the whole method clearer if we say instead:

Return the value of the header *name*, or *default* if there is no header matching *name*.  If there is more than one header with the name *name*, return all of the values joined by ', '.  If 'default' is any iterable other than a single string, its elements are similarly returned joined by commas.