fix type mismatch, convert string to number base 10 and set default i…

[muditjaiswal]

Fixes # checkbox/index.js has tabIndex which is number but is assigned value props.tabIndex which is type string. This issue is potentially causing issue with checkbox focus in other projects where this module is consumed.

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[salesforce-cla[bot]]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Mudit Jaiswal m***@o***.com. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce.com Contributor License Agreement and this Pull Request will be revalidated.

[salesforce-cla[bot]]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Mudit Jaiswal m***@o***.com. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce Inc. Contributor License Agreement and this Pull Request will be revalidated.

[muditjaiswal]

@interactivellama can you review this PR. Thanks!