IE centering bug : sign-in and cover example

[ysds]

The contents are expected to be placed at the horizontal center. But, Windows10 + IE11:

• http://getbootstrap.com/docs/4.0/examples/sign-in/
• http://getbootstrap.com/docs/4.0/examples/floating-labels/

• http://getbootstrap.com/docs/4.0/examples/cover/