add Content-Description to attachment

I used `contentDescription->encodedMIMEHeaderValue()->bytes()` for `mailmime_fields_new_with_data()`. I followed the following from http://www.ietf.org/rfc/rfc2045.txt:

> 8.  Content-Description Header Field
> 
>    The ability to associate some descriptive information with a given
>    body is often desirable.  For example, it may be useful to mark an
>    "image" body as "a picture of the Space Shuttle Endeavor."  Such text
>    may be placed in the Content-Description header field.  This header
>    field is always optional.
> 
>      description := "Content-Description" ":" *text
> 
>    The description is presumed to be given in the US-ASCII character
>    set, although the mechanism specified in RFC 2047 may be used for
>    non-US-ASCII Content-Description values.

Hope I got it right...