Fixed leaking resources when a fetch response body is not consumed

[hammerlscs]

What kind of change does this PR introduce?

Bug fix (fixes https://github.com/supabase/functions-js/issues/65)

What is the current behavior?

I am trying to write some negative tests for my edge functions, for example:

const testSomeFunction = async () => {
  const { data, error } = await supabaseUser.functions.invoke('some-function', {
    body: {
      userId: 'invalid-id'
    }
  })

  assertEquals(error.message, 'Edge Function returned a non-2xx status code')
  assertEquals(data, null)
}

But when running deno test, the test fails with:

error: Leaking resources:
  - A fetch response body (rid 18) was created during the test, but not consumed during the test. Consume or close the response body `ReadableStream`, e.g `await resp.text()` or `await resp.body.cancel()`.

But I cannot access the response to consume/close/cancel/... it.

What is the new behavior?

The response body is cancelled if necessary.