When use border: [{side: "between"}], the between border won't display

Expected Behavior

The between border should display as border: between

Actual Behavior

URL, screen shot, or Codepen exhibiting the issue

https://codesandbox.io/s/grommet-v2-template-forked-ru6opq?file=/index.js

Steps to Reproduce

Your Environment

• Grommet version:
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• Operating System and version (desktop or mobile):