fix: inconsistent encoding in saved sent messages

[st3iny]

Upstream fix at https://github.com/bytestream/Mime/pull/7 or https://github.com/bytestream/Mime/pull/8 (both were tested locally)

TL;DR: Sent email bodies are saved as 7bit quoted-printable but the headers say 8bit.

I switched to the streamed implementation of getRaw() for now. The streamed implementation is also broken but a bit less broken as it defaults to 8bit. I did some bisecting and we used stream=true before https://github.com/nextcloud/mail/pull/9364 but now we use stream=false because the string is cached in the db (outbox table).

How to reproduce?

1. Send some mail with complex UTF-8 chars, e.g. ‘ or nbsp.
2. Wait for it be sent.
3. Have a look at the sent mail in your sent mailbox.
4. Observe weird chars because the mail has inconsistent encoding (headers vs. body).

Screenshots

Before	After

[st3iny]

/backport to stable3.7

[st3iny]

/backport to stable3.6