Only reset the content type to text/plain if it is a multipart currently, otherwise invitation replies get the wrong mime type :/.
Only reset the content type to text/plain if it is a multipart
currently, otherwise invitation replies get the wrong mime type :/.

svn path=/branches/kdepim/proko2/kdepim/; revision=423522

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi