mailutils: remove unnecessary 'multipart/related'

• HTML messages are now sent with the following structure to avoid message truncation in some clients when retrieved over IMAP protocol from an Exchange 2010 server, when the the multipart/related contains only one sub-part (fixes #416):
  
  multipart/alternative
  • text/plain
  • multipart/related
    • text/html
    • image/png
      
      or (when no image):
      
      multipart/alternative
  • text/plain
  • text/html
    
    instead of:
    
    multipart/related
  • multipart/alternative
    • text/plain
    • text/html