Allow the Content-Transfer-Encoding string be NIL (even though it's illegal according to rfc3501)
Allow the Content-Transfer-Encoding string be NIL (even though it's illegal according to rfc3501)

Fixes issue #81

jamie-dainton/MailKit,dsbenghe/MailKit,mganss/MailKit,sstraus/MailKit,matt2003/MailKit,jstedfast/MailKit,rajeshwarn/MailKit,nachocove/MailKit,FubarDevelopment/MailKit,eino-makitalo/MailKit