Responsive NbButton

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
Hello,
I just report a problem with the NbButton component.
This one does not seem responsive with a relatively long text inside.

Here is the rendering with a NbButton:

And here is the rendering with a classic bootstrap button:

**Related code:**
Nebular NbButton:
```

long text long text long text long text long text long text long text long text long text long text

```
bootstrap button:
```

long text long text long text long text long text long text long text long text long text long text

```

**Angular, Nebular**
```
nebular : 4.0.0
angular: 8.0.2
```