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In case it is any help, I tried to find the problem in the
source.  I don't speak python, so I can't fix anything, but
I do speak C, so it's quite readable. :-)

It seems to me that the problem is in Lib/email/Message.py,
in particular in _parseparam and _get_params_preserve.  Both
these functions call strip() on the object several times
(which seems a bit overdone anyway ;-) ), which I presume
removes all whitespace around them.

I think the whitespace should somehow be saved (not
stripping will probably break many programs, so that's not a
good idea), so it can be used again when the header is
written.  set_param should of course also fill this value,
so new params get a defined separation (although the empty
string is quite acceptable).

How this should be implemented I gladly leave to someone who
actually speaks Python. :-)