ssoma-mda: Unfold folding whitespace in message subjects

ssoma-mda: Unfold folding whitespace in message subjects

For the RFC specs on folding whitespace, see [1] and [2].  For the
decision to preserve the original folding in Python headers, here's
R. David Murray's comment from 2011-04-18 [3]:

  I have, by the way, come around to the view that we should never be
  introducing or deleting whitespace except when RFC 2047
  encoding/decoding...we are still deleting it in a couple places, and
  I will address that by and by

[1]: https://tools.ietf.org/html/rfc5322#section-2.2.3
[2]: https://tools.ietf.org/html/rfc5322#section-3.2.2
[3]: http://bugs.python.org/issue1372770#msg133974