Twitter example not working with v41

**Describe the bug**
I recently upgraded to v41 from v40, and this stopped working. Specifically, the following code:

```
// Step #2 - after we received the request tokens, we can start the auth session flow using
// these tokens
const authResponse = await AuthSession.startAsync({
authUrl:
'https://api.twitter.com/oauth/authorize' +
toQueryString(requestTokens),
returnUrl: 'http://localhost:19006/'
});
```

This used to open a new, small window that was Twitter's OAuth page, where I could approve the request. Now, it opens the new, small window, but it just goes straight back to my app. The URL is like:

`http://localhost:19006/start?authUrl=https%3A%2F%2Fapi.twitter.com%2Foauth%2Fauthorize%3Foauth_token%3DREDACTED%26oauth_token_secret%3DREDACTED%26oauth_callback_confirmed%3Dtrue&returnUrl=http%3A%2F%2Flocalhost%3A19006%2F`

It's almost like this `/start` path should apply a redirect, but it is not.

**To Reproduce**
Steps to reproduce the behavior:
1. Clone the `auth-with-twitter` example
2. Run it
3. Observe that the process fails as described above at step 2

**Expected behavior**
The request in the new, small window should redirect to the `authUrl`, e.g. `https://api.twitter.com/oauth/authorize`.

**Screenshots**
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**Desktop (please complete the following information):**
- OS: Windows 10
- Browser Chrome
- Version 90.0.4430.85

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
Add any other context about the problem here.