[Button] Fix boxSizing when not rending a native button

[oliviertassinari]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

The native <button /> inherit from a default box-sizing: border-box model from the browser style. That's not true with a <a /> for instance. As always when the box dimension can vary depending on the box-sizing model, we reset it.