Fix #5929. Set default prop when prop is undefined while cloning

[arjunsharma]

Set default props if a passed in prop is undefined while using cloneElement, the same as what happens when doing createElement.

[jimfb]

Thanks for the PR! We have already accepted #5997 and will merge once we've determined that it's safe and doesn't require a migration plan. For that reason, I'm going to close out this PR in favor if #5997