Step::replace_zero actually replaces it with one

Repro (nightly 2017-04-22):
```rust
#![feature(step_trait)]
use std::iter::Step;
fn main() {
    let mut x = 3;
    x.replace_zero();
    assert_eq!(x, 0);
}
```
Actual:
```
assertion failed: `(left == right)` (left: `1`, right: `0`)
```