Underscore in `format!` named substitutions causes "unterminated format string" error

``` rust
fn main() {
    format!("{foo_bar}", foo_bar = "bad underscore")
}
```

```
underscore.rs:2:12: 2:23 error: unterminated format string
underscore.rs:2     format!("{foo_bar}", foo_bar = "bad underscore")
                            ^~~~~~~~~~~
error: aborting due to previous error
```
