chore: replace Vec::write_all usage with Vec::extend_from_slice
chore: replace Vec::write_all usage with Vec::extend_from_slice

The underlying implementation simply calls extends_from_slice anyway,
but this removes the error that would never happen
https://doc.rust-lang.org/1.45.2/src/std/io/impls.rs.html#389-393

amousset/lettre,amousset/rust-smtp,lettre/lettre,amousset/lettre