NbRoleProvider getRole() is cached

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I have 2 accounts 1 with role "admin" and another with role "user", when I am logged in with "admin" role account then, log out and log in with an account which has role "use" I still see NbRoleProvider "getRole" returning "admin' role.
After reloading the page it stars returning the correct role.

**Expected behavior:**
After log out and log in with different role accounts NbRoleProvider.getRole() method should return the correct role.

### Other information:

**npm, node, OS, Browser**
```
node version v10.16.3
npm version 6.9.0
OS Windows 10
Browser Chrome

**Angular, Nebular**
@angular 8.0.3
@nebular 4.4.0