fix ICS text encoding using 8bit

Created by: adipasquale

cf https://trello.com/c/EoXzM9kK/826-v%C3%A9rifier-la-bonne-gestion-des-accents-dans-les-invitations-ical

the bug only reproduces when sending through SIB's SMTP. I enabled it locally by copying prod's config. This fix seems to be enough.

I really don't understand much to what's happening here, but I think the default is base64 but it's not perfect for plain text files like ICS ... I don't really know 😬

https://en.wikipedia.org/wiki/MIME#Content-Transfer-Encoding