Improve Input group border radius

Created by: ysds

There are input group border radius issues with latest CSS (after "Rewrite input group component" #25020).

1. Right corner of input are rounded
2. Multiple dropdown trigger's corner are incorrect. Usage of multiple button addons are not described in the documents, but they are correctly display in beta 2.

Demo:

• beta 2: https://jsfiddle.net/h3aa6bg4/
• current: https://jsfiddle.net/m3y46k7w/

And following limitation can be removed with selector improvement. Please see my PR. https://github.com/twbs/bootstrap/blob/2416801c7ea6e67d728096e0ed74568f6c742fc7/docs/4.0/migration.md#L38 Thanks.