RadioGroup Preview should work based on index.

[mannycarrera4]

🐛 Bug Report

In Radio Main, we map over children and give it a default value : https://github.com/Workday/canvas-kit/blob/master/modules/react/radio/lib/RadioGroup.tsx#L74

In preview if you don't provide a value it doesn't default by using the index of the radio input. Should this work by default? Technically this is a change in the API. In Preview, you have to explicitly define a value in order for the radio to work.