bad rfc2047 encoding

[Dave Love]

In Emacs 21.2, I see

(with-temp-buffer
  (insert "To: Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai Großjohann)
")
  (rfc2047-encode-message-header)
  (buffer-string))
  => "To: Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai =?iso-8859-1?q?Gro=DFjohann)?=
"

which is an invalid encoding according to §5 of rfc2047, and Exim
refuses to deliver the result:

  Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai =?iso-8859-1?q?Gro=DFjohann)?=: 
malformed address: ?= may not follow Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai 
=?iso-8859-1?q?Gro=DFjohann)

I'm surprised this hasn't been noticed and fixed before, but I think
it needs fixing for 21.3 by someone more familiar with the standards
and the logic in rfc2047.el.