Fixed crash when message does not contain sender or recipient
add Content-Description to attachment

- using contentDescription->encodedMIMEHeaderValue()->bytes() for mailmime_fields_new_with_data()
Merge pull request #344 from qvacua/master

add Content-Description to attachment
Added assert in provider parser
Improved Content-Description support and fixed crash (Fixed #350)
Additional error descriptions
Merge remote-tracking branch 'upstream/master'