nbSelect not update after nbOption change

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
selected value not update

**Expected behavior:**
selected value should update

**Steps to reproduce:**
set value of 'id' and then change value of 'datas', UI no update

**Related code:**

```

-- Select --
{{data.name}}

```

### Other information:

**npm, node, OS, Browser**
```
node: 10.15.3
npm: 6.4.1
OS: Linux
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 8.0.0
Nebular: 4.1.2
```