Improve styling of btn-group class

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

`btn-group` styles don't apply properly when it's just one item.

**Expected behavior:**

Styles should be applied properly regardless of the number of items in the group.

**Steps to reproduce:**

Install the `@nebular/bootstrap` package and configure it.

Create a `div.btn-group>button.btn.btn-primary`

**Related code:**

https://stackblitz.com/edit/nebular-btn-group-bug

```

```

### Other information:

**npm, node, OS, Browser**
```
npm: 6.9.0
node: 10.16.0
OS: Windows 10
```

**Angular, Nebular**
```
{
"name": "jvminzkz.github",
"version": "0.0.0",
"private": true,
"dependencies": {
"@angular-devkit/build-angular": "0.801.1",
"@angular/animations": "8.1.1",
"@angular/cdk": "8.0.2",
"@angular/common": "8.1.1",
"@angular/compiler": "8.1.1",
"@angular/compiler-cli": "^8.0.0-beta.0 || ^8.1.0-beta.0 || ^8.2.0-beta.0 || ^8.3.0-beta.0 || ^8.4.0-beta.0 || >=9.0.0-beta < 9",
"@angular/core": "8.1.1",
"@angular/forms": "8.1.1",
"@angular/platform-browser": "8.1.1",
"@angular/platform-browser-dynamic": "8.1.1",
"@angular/router": "8.1.1",
"@nebular/bootstrap": "^4.4.0",
"@nebular/theme": "4.1.1",
"@types/jasmine": "2.8.16",
"@types/jasminewd2": "2.0.6",
"@types/node": "8.9.5",
"bootstrap": "^4.0.0",
"core-js": "2.6.9",
"jquery": "1.9.1 - 3",
"nebular-icons": "1.1.0",
"popper.js": "^1.14.7",
"rxjs": "6.5.2",
"tslib": "1.10.0",
"typescript": ">=3.1 < 3.5",
"zone.js": "0.9.1"
},
"scripts": {
"ng": "ng",
"start": "ng serve",
"build": "ng build",
"test": "ng test",
"lint": "ng lint",
"e2e": "ng e2e"
},
"devDependencies": {
"@angular-devkit/build-angular": "~0.10.0",
"@angular/cli": "~7.0.2",
"@angular/compiler-cli": "~7.0.0",
"@angular/language-service": "~7.0.0",
"@types/node": "~8.9.4",
"@types/jasmine": "~2.8.8",
"@types/jasminewd2": "~2.0.3",
"codelyzer": "~4.5.0",
"jasmine-core": "~2.99.1",
"jasmine-spec-reporter": "~4.2.1",
"karma": "~3.0.0",
"karma-chrome-launcher": "~2.2.0",
"karma-coverage-istanbul-reporter": "~2.0.1",
"karma-jasmine": "~1.1.2",
"karma-jasmine-html-reporter": "^0.2.2",
"protractor": "~5.4.0",
"ts-node": "~7.0.0",
"tslint": "~5.11.0",
"typescript": "~3.1.1"
}
}
```