std::io::Chain incorrectly assumes that Ok(0) returned from read indicates EOF.

For example:

use std::io::{Cursor, Read};

fn main() {
    let mut reader = Cursor::new(
        b"A".to_owned()
    ).chain(
        Cursor::new(b"B".to_owned())
    );

    reader.read(&mut []).unwrap();
    let mut s = String::new();
    reader.read_to_string(&mut s).unwrap();
    assert_eq!("AB", s);
}

Results in:

thread 'main' panicked at 'assertion failed: `(left == right)` (left: `"AB"`, right: `"B"`)'