So astoundingly, this change has deep implications.  The upshot is that
it's difficult to fix this so that headers look nice for e.g. Subject
headers, but so that splitting and wrapping work as expected for e.g.
machine readability of Received headers.

After discussion with other sprinters, I'm committing a change to Python
2.7 to fix this, but I am not back porting the change to 2.6.  I think
we should Do It Right for Python 3.1 but this requires (IMO) API changes.

r70772