Before submitting a pull request, please make sure you have at least performed the following:

 - [] read and followed the [CONTRIBUTING.md](https://github.com/ng-bootstrap/ng-bootstrap/blob/master/CONTRIBUTING.md) guide.
 - [] built and tested the changes locally.
 - [] added/updated any applicable tests.
 - [] added/updated any applicable API documentation.
 - [] added/updated any applicable demos.