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

[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: angular-ui/bootstrap#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

[patrickhlauke]

Not going to get any fixes for v3, but relabelled as v4 as it happens there as well

[patrickhlauke]

closing this issue here - tracking the problem in the meta-issue #28481