add Basic auth header to the request

This is needed as per the [documentation](https://developer.paypal.com/docs/api/#grant-token-from-authorization-code)

```javascript
curl -v https://api.sandbox.paypal.com/v1/identity/openidconnect/tokenservice \
-u "{clientId}:{secret}" \
-d "grant_type=authorization_code
    &code={authorizationCode}
    &redirect_uri=http://example.com/myapp/return.php"
```

The curl example passes on Basic Auth 


fixes paypal/rest-api-sdk-nodejs#26