Class modal-dialog-centered does not work on IE 11

Created by: Webdude737

This might an issue with Flexbox and IE 11. In any case, opening up the modal does not vertically center it.

Easy to Reproduce: In IE11, go to https://getbootstrap.com/docs/4.0/components/modal/ and click on the "Launch demo modal" button in the section "Vertically centered".

IF there is a workaround class or code, please advise. Thanks.