#14380: Make actual default match docs, fix __init__ order.

#14380: Make actual default match docs, fix __init__ order.

Éric pointed out that given that the default was documented as None, someone
would reasonably pass that to get the default behavior.  In fixing the code to
use None, I noticed that the change to _charset was being done after it had
already been passed to MIMENonMultipart.  The change to the test verifies that
the order is now correct.