nbDatePicker returned value not parsed

### Issue type

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When using formatted nbDatePicker with moment or date-fns with angular reactive form returned value of that input is always in full date format regardless the passed format to nbDatePicker.

**Expected behavior:**
nbDatePicker should parse value to desired format.

**Steps to reproduce:**

after exiaming mydate value is always in this format "Wed Jan 12 2022 00:00:00 GMT 0300"

### Other information:

**npm, node, OS, Browser**
```
Node, npm: `v14.18.2` and `6.14.15`
OS: macOS (Big Sur)
Browser: Chrome
```

**Angular, Nebular**
```
@nebular/theme@6.0.0
@angular/core" "9.0.0
```