AbortSignal should abort response body

[mwatson2]

According to fetch http-network-fetch, Step 7, reception of the response body should be aborted if the AbortSignal provided to the original call to fetch fires. In node-fetch, this is implemented here, by destroying the stream. Notably, this happens outside the Response object.

Unless I missed it, fetch-mock does not do this and the AbortSignal only aborts the initial step of resolving the promise returned by fetch with the Response object.

I'm happy to provide a PR to resolve this, if the issue is valid.