Self signed certificates
Trying to use this library with the OAuth2.0/IODC server hosted with self signed certificate and getting this error on callback/verify call
```
error=unable%20to%20verify%20the%20first%20certificate
```

Is there a way to set something like `new https.Agent({rejectUnauthorized: false}` to the underlying request library?

Thank you in advance, pretty nice library btw!