Canceling requests with TypeScript

Since the `cancel/abort` function is added runtime to the `Promise` returned by `.send()` in [`http-client.js`](https://github.com/aurelia/http-client/blob/master/src/http-client.js#L107), the TypeScript compiler doesn't know about its existence.

Start at src/http-client.js around line 107 and inspect how the runtime cancel/abort function is added to the Promise returned by .send(). Check the TypeScript-facing definition or compiler path for that return value; done when TypeScript recognizes the available cancellation function without an error.

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