[2.0.0] Handling a ReadableStream in response

```
export const fetchImage = imageName => ({
[CALL_API]: {
endpoint: `/resources/${imageName}.jpg`,
method: 'GET',
types: [
IMAGE_REQUEST,
{
type: IMAGE_SUCCESS,
meta: { imageName },
payload: (action, state, res) => res.arrayBuffer()
},
IMAGE_FAILED],
},
});
```
Im trying to retrieve an image as an arraybuffer (it is protected with a jwt token so I need to fetch it myself and add the raw data to the ).
But the readable stream returns a Promise (res.arraybuffer() https://jakearchibald.com/2015/thats-so-fetch/) and I'm stuck on how to handle that further down the road.

I tried

```
case IMAGE_SUCCESS: {
const mimetype = 'image/jpg';
const arrayBufferPromise = action.payload;
const imageName = action.meta.imageName;

return {
...state,
[imageName]: arrayBufferPromise.then(arrayBuffer => encodeImage(arrayBuffer, mimetype)),
loading: false,
};
}
```
but without success.