mailutils: remove unnecessary 'multipart/related'
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

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio