fix(modal): shift+tab without focused element within modal

Created by: Hardhik

Issue can be validated here:

1. Navigate to http://getbootstrap.com/javascript/
2. Search for "Launch demo modal" and click on that. Now use shift+tab, nothing happens.

There was a similar bug in Angular-ui's bootstrap which got fixed: https://github.com/angular-ui/bootstrap/pull/5294

This should be done even in this library too.

I have tried on: Operating system and version Windows, Browser and version Chrome: 55.0.2883.87