Default view of nb-select is middle of the list

### Issue type

**I'm submitting a ...**

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
The overlay view of an nb-select defaults to the middle of the list when first opened. Subsequent views reset to the top.

**Expected behavior:**
The initial overlay should focus on the first item.

**Steps to reproduce:**
Create an nb-select with enough options to cause scrolling.

**Related code:**
The example already shows this behaviour:
https://akveo.github.io/nebular/example/select/select-groups.component

### Other information:

**npm, node, OS, Browser**
```
OS: macOS Sierra
Browser: Chrome
```

**Angular, Nebular**
```
Angular 8
Nebular 4.1.3
```