NbAuthModule Login use OPTIONS instead of POST

### Issue type

**I'm submitting a ...** (check one with "x")

* [x ] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I'm implementing a Login system which uses `NbPasswordAuthStrategy` with a Django Backend (+ Knox). The login fails even with correct data and checking the logs of my backend I see that the request had arrived but with OPTION http method instead of POST.
```

NbAuthModule.forRoot({
strategies: [
NbPasswordAuthStrategy.setup({
name: 'email',
baseEndpoint: 'http://127.0.0.1:8000',
login: {
// ...
endpoint: '/login/',
method: 'POST',

redirect: {
success: '/home/',
failure: '',
},
},
}),
],
forms: {},
},
}),
```