Menu does not display active class.

### Issue type

menu does not include an active class within the link.

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

* [X] bug report
* [ ] feature request

### Issue description
menu does not include an active class within the link.

**Current behavior:**

when selecting a menu the active class is not displayed

**Expected behavior:**

Let the nebular documentation match the active menu with the "active" class inside the tag a

**Steps to reproduce:**

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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