Sidemenu Menu-Item URL (target='blank') Selection is getting highlighted

### Issue type

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

* [ ] bug report

### Issue description

**Current behavior:**
If we click on Menu Item URL which opens in new browser tab getting selected.
Menu Item is getting selected when user clicks on a URL menu item with target _blank. This is resulting in unexpected behavior of the menu.

In the below typescript code of Menu, when user selects Dashboard and then selects FAQs under Halp & Support, Dashboard is still shown but FAQs is highlighted.
The app page URL is not changing (Dashboard URL) and current Menu Item (Dashboard) selection is going off.

**Expected behavior:**

The Menu Item which is already selected , should not change selection if we click on Menu Item URL (target='_blank').

**Steps to reproduce:**

**Related code:**
import { environment } from "environments/environment";

export const MENU: any = [
{
title: 'Dashboard',
icon: 'fas fa-th',
expanded: false,
home: true,
link: '/pages/dashboard/dashboard',
hidden: true,
},
{
title: 'Help & Support',
icon: 'far fa-question-circle',
expanded: false,
home: false,
hidden: false,
children: [
{
title: 'Installation Guide',
url: environment.storageLink + 'InstallationGuide.pdf',
target: '_blank',
hidden : false,
},
{
title: 'FAQ\'s',
url: environment.storageLink + 'FAQ.pdf',
target: '_blank',
hidden: false,
},
],
},
];

### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
Angular : 7.0.3
Nebular : 3.5.0

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