NbSidebar: menu item toggled when expanding sidebar

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description
When sidebar is compacted, clicking on menu item with submenu causes menu item toggling.

**Current behavior:**
Menu item 'expanded' state changed after each click.

**Expected behavior:**
Clicked menu item should always be expanded.

**Steps to reproduce:**
1. Go to [ngx-admin](http://akveo.com/ngx-admin/).
2. Shrink viewport to the size when sidebar is compacted.
3. Click on any menu item which has submenu.
4. Collapse sidebar.
5. Repeat steps 3 and 4 several times.

Every time menu item has a different state.

**Related code:**
https://github.com/akveo/nebular/blob/7542d5ed11a6556b7a91c4209e58a878daed37fa/src/framework/theme/components/sidebar/sidebar.component.ts#L296
https://github.com/akveo/nebular/blob/7542d5ed11a6556b7a91c4209e58a878daed37fa/src/framework/theme/components/menu/menu-item.component.html#L36

### Other information:

**npm, node, OS, Browser**
```
any
```

**Angular, Nebular**
```
@nebular/theme: "2.0.0-rc.10"
```