Use type button by default in Tab component

[rubenmoya]

This PR fixes #165, it was first fixed in 5f75dd5 but then broken in a2ab2be.

I've added a couple of tests but I'm not sure if they're what you expect so feedback is welcome!

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[chaance]

The reason this was removed to begin with is because this check isn't reliable. Comp might be a styled component, or a wrapper component of some kind, in which case we'd lose that default setting. Of course if we pass the attribute along to a non-button element then we have invalid HTML. May still be worth it to have either way.

[rubenmoya]

Maybe we could keep the check, since it would cover at least some cases, and add some information in the docs about the edge cases?