fix: use write_all() to write space after a tag

[link2xt]

write() should not return Ok without writing anything, but the documentation is not clear on this
and underlying implementation may be wrong
as IMAP can be run over various streams,
e.g. compressed, encrypted, proxied.
It is safer to use write_all() everywhere,
even when we write a single byte.

[Hocuri]

The tokio implementation for write_all() even returns Ready(Err(...)) if 0 bytes were written even though there is still data in the buffer:

impl<W> Future for WriteAll<'_, W>
where
    W: AsyncWrite + Unpin + ?Sized,
{
    type Output = io::Result<()>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        let me = self.project();
        while !me.buf.is_empty() {
            let n = ready!(Pin::new(&mut *me.writer).poll_write(cx, me.buf))?;
            {
                let (_, rest) = mem::take(&mut *me.buf).split_at(n);
                *me.buf = rest;
            }
            if n == 0 {
                return Poll::Ready(Err(io::ErrorKind::WriteZero.into())); // <-- Here
            }
        }

        Poll::Ready(Ok(()))
    }
}

so, if the stream implementation is buggy and returns 0, then the change here won't help anything.

I'll nevertheless approve, since nothing speaks against using write_all here, and it may prevent errors in the future where someone tries to write more than one byte using write().

[link2xt]

There is also async std still supported next to tokio.