Input addon incorrect vertical alignment with flex enabled

Created by: KyleNeedham

When enable-flex is set to true there is no input-addon vertical alignment.

bootstrap: ^4.0.0-alpha.4
OS: Ubuntu 16.0.4 LTS
Browsers:
    Chrome 54.0.2840.59 (Official Build) (64-bit)
    Firefox 49.0

Example with flex enabled

enable-flex: true Reduced test case: https://jsbin.com/fimigirefu/edit?html,output I had to embed the built CSS into this bin, since I couldn't find a CDN with flex enabled. It was built using the 4.0.0-alpha.4 bootstrap release and no other sass included.

Example without flex enabled

enable-flex: false Reduced test case: https://jsbin.com/yoyagecifo/edit?html,output