NbPasswordAuthStrategy not calling refresh endpoint

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

I am developing a web app based on ngx-admin. I cloned the basic branch and started modifying the Authentication system to make it work with my auth server.
My auth server sends back at login a JWT and a HTTPOnly cookie with a refresh token. I have then a refresh_token API to refresh the JWT with the refresh token.
I am using NbPasswordAuthStrategy. I saw there's a way to automatically refresh the token with the strategy as seen here https://akveo.github.io/nebular/docs/auth/nbpasswordauthstrategy#nbpasswordauthstrategy

**Current behavior:**

JWT works perfectly. I created a simple interceptor to add withCredentials: true to every request so I can send the httponly cookie at every request. This works perfectly too. But...
I configured the refreshToken property in the NbStrategy but it is never called, so obviously I never get my token refreshed. Am I missing something or something is not working?

**Expected behavior:**
Refresh token endpoint gets called automatically

**Related code:**

This is my strategy

```
NbAuthModule.forRoot({
strategies: [
NbPasswordAuthStrategy.setup({
{
baseEndpoint = '/api/auth/';
login: {
endpoint: 'login',
method: 'post',
redirect: {
success: '/home',
failure: null,
},
};
logout: {
endpoint: 'logout',
method: 'post',
redirect: {
success: '/login',
failure: null,
},
};
refreshToken: {
endpoint: 'refresh_token',
method: 'post',
requireValidToken: true,
redirect: {
success: null,
failure: null,
},
};
token: {
class: NbAuthJWTToken,
key: 'accessToken',
};
...........

```