Vertical text alignment of select.form-control text within .input-group is not centered

Created by: bdrelling

Operating System: Mac OS Sierra 10.12.3 Browser: Chrome 55.0.2883.95 (64-bit) Bootstrap: v4.0.0-alpha.6

Issue Description:

When using select.form-control within an .input-group, text is vertically aligned to the top of the box rather than vertically aligned to the middle.

Screenshot:

Reduced Test Case:

<div class="input-group">
  <span class="input-group-addon">@</span>
  <input type="text" class="form-control" placeholder="Username">
</div>

<div class="input-group">
  <span class="input-group-addon">@</span>
  <select class="form-control"><option>Username</option></select>
</div>

Additional Comments:

I'm unable to test in other browsers/operating systems at this time but can amend these results later. If someone can do so in the meantime, I'd appreciate it.

Also, any typical JS Bin-type site we're using for Bootstrap 4 right now? I didn't see one in the issues I peeked at but, if it exists, I want to be aware of it moving forward to make it easier for everyone.