Safari does not include cookies in JSONP requests

### How to reproduce these conditions

**Sample name or URL where you found the bug**

[firebase/functions-samples/spotify-auth](https://github.com/firebase/functions-samples/tree/Node-8/spotify-auth), but the other auth examples may be affected too.

**Failing Function code used (including require/import commands at the top)**

[spotify-auth/functions/index.js](https://github.com/firebase/functions-samples/blob/Node-8/spotify-auth/functions/index.js#L62)

```js
const functions = require('firebase-functions');
const cookieParser = require('cookie-parser');
[...]
exports.token = functions.https.onRequest((req, res) => {
try {
cookieParser()(req, res, () => {
console.log('Received verification state:', req.cookies.state);
console.log('Received state:', req.query.state);
if (!req.cookies.state) {
throw new Error('State cookie not set or expired. Maybe you took too long to authorize. Please try again.');
} else if (req.cookies.state !== req.query.state) {
throw new Error('State validation failed');
}
[...]
```

**Steps to set up and reproduce**

Setup the Spotify-auth example and use Safari (Version 12.0 14606.1.36.1.9) to test it.

### Debug output

**Errors in the [console logs](https://console.firebase.google.com/project/_/functions/logs?search=&severity=DEBUG)**

`State cookie not set or expired. Maybe you took too long to authorize. Please try again.`

The `req.cookies` object is empty.

### Expected behavior

No exception.

### Actual behavior

As far as I can see, Safari does not include cookies in JSONP request. It works fine in Chrome though.