[Layout] header with both fixed and subheader attributes

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When the page is small enough the header scrolls with the main layout content

**Expected behavior:**
When setting both `fixed` and `subheader` to a layout header I would expect it to be fixed on top instead of scroll with the page when using a mobile browser

**Steps to reproduce:**
https://stackblitz.com/edit/github-hin584

### Other information:

**npm, node, OS, Browser**
```
Node 11.11.0, npm 6.9.0, OSX Mojave, Chrome 73
```

**Angular, Nebular**
```
Angular 7.2.0, Nebular 3.4.0
```