Add support for duplicate keys when calling `.append()` with Arrays.
Add support for duplicate keys when calling `.append()` with Arrays.

Example:

```js
body.append({
    file: [
        new multipart.RequestField('foo', 'text/plain'),
        new multipart.RequestField('bar', 'text/plain')
    ]
});
```

synacorinc/maltypart,synacorinc/maltypart