Variable name with internal double-underscore gets snake case warning, but leading/trailing does not

``` rust
#![allow(unused_variables)]

fn main() {
    let x__y = 0u;

    let __xy = 0u;
    let xy__ = 0u;
}
```

```
doubleunder.rs:4:9: 4:13 warning: variable `x__y` should have a snake case name such as `x_y`, #[warn(non_snake_case)] on by default
doubleunder.rs:4     let x__y = 0u;
                         ^~~~
```

It seems slightly weird that we are inconsistent.
