`read_until` is just doing a bytewise comparison. This means the following program prints `xyå12`, not `xy`, which it should if it was actually checking chars.

```rust
fn main() {
    do io::with_str_reader("xyå12") |r| {
        io::println(r.read_until('å', false));
    }
}
```

This patch makes the type of read_until match what it is actually doing.

Fix false negative in `option_as_ref_deref`

Closes rust-lang#5927

changelog: Fix false negative in `option_as_ref_deref`