v4 clearfix on .card-{block,header,footer} breaks flex layouts

Created by: ju1ius

In latest v4-dev branch, the clearfix mixin is added on all the card subcomponents. If the subcomponent is added a flex display, the :after pseudo added by the mixin adds unwanted spacing at the end of the flex block's main axis.

.card-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

<div class="card">
  <img class="card-img-top" src="http://loremflickr.com/320/180/space-oddity">
  <div class="card-block">
    <h2 class="card-title">Space Oddity</h2>
    <div class="card-subtitle">David Bowie ✝</div>
  </div>
  <div class="card-footer">
    <span>Release: 1969</span>
    <div class="btn-group">
      <div class="btn btn-sm btn-primary"><i class="fa fa-play"></i></div>
      <div class="btn btn-sm btn-warning"><i class="fa fa-star"></i></div>
    </div>
  </div>
</div>

I think the mixin should be guarded by a @if(not $enable-flex) block;