BufferedReader's fill() doesn't fill the buffer

[Florob]

The fill() method of BufferedReader does not fill the internal buffer, unless the buffer is completely empty. This does not match the description in the Buffer trait. One case where this can cause problems is when using read_char(), e.g.:

fn main() {
    let mut s = "a".repeat(64*1024-1);
    s.push_char('ß');

    let rdr = std::io::mem::MemReader::new(s.into_bytes());
    let mut rdr = std::io::buffered::BufferedReader::new(rdr);

    rdr.fill();
    rdr.consume(64*1024-2);

    println!("{}", rdr.read_char()); // Some('a')
    println!("{}", rdr.read_char()); // None, should be Some('ß')
    println!("{}", rdr.read_char()); // None
}