Where to place nb-sidebar-header within nb-layout in v4

### Issue type

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

* [X] bug report
* [ ] feature request

### Issue description

**Current behavior:**
Below code displays the sidebar within the scroll container. It leaves empty space equal to the height of the main header

**Expected behavior:**
But as per the documentation, it should be displayed on top of the scroll container.

**Steps to reproduce:**
Run the app with above layout code.

**Related code:**

```

My Sidebar Header

```

### Other information:

**npm, node, OS, Browser**
```

```

**Angular, Nebular**
```
"dependencies": {
"@angular/animations": "~8.0.0",
"@angular/cdk": "^8.0.0",
"@angular/common": "~8.0.0",
"@angular/compiler": "~8.0.0",
"@angular/core": "~8.0.0",
"@angular/forms": "~8.0.0",
"@angular/platform-browser": "~8.0.0",
"@angular/platform-browser-dynamic": "~8.0.0",
"@angular/router": "~8.0.0",
"@nebular/bootstrap": "^4.1.0",
"@nebular/eva-icons": "4.1.0",
"@nebular/theme": "^4.1.0",
"rxjs": "~6.5.2",
"tslib": "^1.9.0",
"zone.js": "~0.9.1"
}
```