NbCheckbox does not uncheck when checked = false

### Issue type

**I'm submitting a ...**
* [ x ] bug report
* [ ] feature request

### Issue description

**Current behavior:**
NbCheckbox does not accept value (warning says it will be removed on 5.0.0, I'm running 5.1.0 value still remains there) and [checked]="false" does not work. I've tried to use [checked]="model.prop" where model.prop is a boolean and it doesn't work checkbox is always checked.

**Expected behavior:**
When [checked]="false" the checkbox should be unchecked

**Steps to reproduce:**
Create a checkbox in any form and add [checked]="false".

**Related code:**
```

My property value: {{ model.prop }}

```

-->

### Other information:

**npm, node, OS, Browser**
```
Angular CLI: 9.1.8
Node: 12.16.1
OS: darwin x64

Angular: 9.1.11
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router
Ivy Workspace: Yes

```

**Angular, Nebular**
```
"@nebular/auth": "^5.1.0",
"@nebular/eva-icons": "5.1.0",
"@nebular/firebase-auth": "^5.1.0",
"@nebular/security": "^5.1.0",
"@nebular/theme": "^5.1.0",
```