#19772: Do not mutate message when downcoding to 7bit.

#19772: Do not mutate message when downcoding to 7bit.

This is a bit of an ugly hack because of the way generator pieces together the
output message.  The deepcopys aren't too expensive, though, because we know it
is only called on messages that are not multiparts, and the payload (the thing
that could be large) is an immutable object.

Test and preliminary work on patch by Vajrasky Kok.