ssoma-mda: Fix RFC-2047 decoding for partially-encoded strings

ssoma-mda: Fix RFC-2047 decoding for partially-encoded strings

Avoid:

  TypeError: sequence item 0: expected str instance, bytes found

When an RFC-2047-encoded string contains both an unencoded and an
encoded section.  For example:

  >>> import email.header
  >>> email.header.decode_header('Keld =?ISO-8859-1?Q?J=F8rn_Simonsen?=')
  [(b'Keld ', None), (b'J\xf8rn Simonsen', 'iso-8859-1')]

returns the decoded string in bytes but no charset information for the
first chunk.  I'm not sure what the default charset for header values
is, but RFC 2047 sets itself up to deal with non-ASCII header values
[1], so I'm guessing it's ASCII ;).

[1]: http://tools.ietf.org/html/rfc2047#section-1