fix: `on` does not support multiple fields
fix: `on` does not support multiple fields

weirdly though, this works:

```js
const foo = { a: 'a' };

console.log(foo['a']); // => 'a'
console.log(foo[['a']]); // => 'a' // yup!
console.log(foo[[['a']]]); // => 'a' // yup! :D
```

joelmukuthu/knorm