Accept the empty array as a single argument

```js
> zipmap([{ key: 'foo', value: 'bar' }].filter(function (item) {
    return item.key == 'foo';
  }))
{ foo: 'bar' }
> zipmap([{ key: 'foo', value: 'bar' }].filter(function (item) {
    return item.key == 'fo';
  }))
TypeError: Expected vals to be an array
```