Fixes #120: CancellationToken.is does not check for undefined

[siegebell]

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[msftclas]

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[msftclas]

Hi @siegebell, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;

[dbaeumer]

This is already fix in master using the following code:

	export function is(value: any): value is CancellationToken {
		let candidate = value as CancellationToken;
		return candidate && (candidate === CancellationToken.None
			|| candidate === CancellationToken.Cancelled
			|| (Is.boolean(candidate.isCancellationRequested) && Is.defined(candidate.onCancellationRequested)));
	}

[dbaeumer]

Thanks for the test case which I merged in by hand.