Handle sane `customFields`

The most logical way to set custom fields would be:

```js
customFields: {
  key1: value1,
  key2: value2
}
```

However, because we just pass the data straight through, the format must be:

```js
customFields: [
  {
    key: key1,
    value: value1
  },
  {
    key: key2,
    value: value2
  }
]
```

We should support both formats.