Modal should add "modal-open" class to body

[BorisNaguet]

Hello,

As explained here:
https://getbootstrap.com/docs/4.0/components/modal/#usage

When the modal is opened, there should be a "modal-open" class added to the body element.

See js version code here:
https://github.com/twbs/bootstrap/blob/v4-dev/js/src/modal.js#L141

and it should be removed on close:
https://github.com/twbs/bootstrap/blob/v4-dev/js/src/modal.js#L322

Thanks

[BorisNaguet]

It has an influence when the modal is longer than the screen, like :
https://getbootstrap.com/docs/4.0/components/modal/#scrolling-long-content