Fails when size is 1.

Hi, I tried do create a monogram with this, and it fails like this:

```rust
let grams: Vec<_> = "one two three".split(' ').ngrams(1).collect();
println!("{:?}", grams);
```

```
[["one"], ["one", "two"], ["two", "three"]]
```

I expected this to return

```
[["one"], ["two"], ["three"]]
```