Don't encode square brackets

Per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI it seems we should not encode square brackets, so that...

```
Arg.stringify({foo: ["bar", "baz"]});
// foo[0]=bar&foo[1]=baz
```

rather than...

```
Arg.stringify({foo: ["bar", "baz"]});
// foo%5B0%5D=bar&foo%5B1%5D=baz
```

Sound right? Happy to provide a PR...
