(mime-edit-decode-message-in-buffer): Call

(mime-edit-decode-message-in-buffer): Call
`mime-edit-decode-single-part-in-buffer' with return value of
`mime-read-Content-Disposition'; don't modify the header if the buffer is
narrowed to a part.
(mime-edit-decode-single-part-in-buffer): Accept the optional arg
`content-disposition' and inherit it into the editing buffer.