Some data is dropped on buffer boundary crossing when implementing minimal custom Stream of Lines over BufReader.

I tried to implement minimalist `Stream` over lines in a `BufReader` using `read_line` method, but it drops some data when boundary of its buffer is crossed.
Using `async_std::io::Lines` does not reproduce the issue, as well as calling `read_line` in a loop does not reproduce it either.

Here is the code:
```rust
use std::{
pin::Pin,
task::{Context, Poll},
};

use async_std::{
fs::File,
io::{prelude::*, stdout, BufReader, Result},
prelude::*,
stream::Stream,
task,
};

fn main() -> Result<()> {
task::block_on(run())
}

async fn run() -> Result<()> {
let input = BufReader::with_capacity(284, File::open("pattern.txt").await?);
let mut lines = Lines::new(input);

while let Some(line) = lines.next().await {
stdout().write_all(line?.as_bytes()).await?;
}

Ok(())
}

pub struct Lines {
file: BufReader,
}

impl Lines {
pub fn new(file: BufReader) -> Self {
Self { file }
}
}

impl Stream for Lines {
type Item = Result;

fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll> {
let mut buf = String::new();
let mut line = self.file.read_line(&mut buf);
match Pin::new(&mut line).poll(cx) {
Poll::Pending => Poll::Pending,
Poll::Ready(Err(e)) => Poll::Ready(Some(Err(e.into()))),
Poll::Ready(Ok(0)) => Poll::Ready(None),
Poll::Ready(Ok(_)) => Poll::Ready(Some(Ok(buf))),
}
}
}
```

Here is the content of the file `patttern.txt`:
```
00 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
01 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
02 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
03 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
04 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
05 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
06 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
07 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
08 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
09 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
```

Here is the output of the sample program:
```
00 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
01 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
02 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
03 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
05 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
06 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
07 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
3456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
09 | 0123456789-ABCDEFGHIJKLMNOPQRSTUVWXYZ+abcdefghijklmnopqrstuvwxyz
```

Notes:
* Length of a single line is 70 bytes.
* Buffer size is set to 284 bytes (4 lines + 4 bytes).
* When buffer boundary is crossed (after first 4 lines), next 4 bytes are dropped (the remainder of the buffer) and following line contains only remaining 66 bytes.