Response body uses a `NopCloser`, prevents detecting `http: read on closed response body` errors

[zacharyarnaise]

gock uses a ioutil.NopCloser for its response bodies:

gock/responder.go

Lines 107 to 111 in 0c43888

	// createReadCloser creates an io.ReadCloser from a byte slice that is suitable for use as an
	// http response body.
	func createReadCloser(body []byte) io.ReadCloser {
	return ioutil.NopCloser(bytes.NewReader(body))
	}

This makes it very hard to catch faulty code which would try to read on a response's body after it's been closed:

func TestHTTP(t *testing.T) {
	gock.Intercept()
	gock.Observe(gock.DumpRequest)

	gock.New("https://httpbin.org").
		Get("/get").
		Reply(200).
		JSON(map[string]string{"foo": "bar"})

	resp, err := http.Get("https://httpbin.org/get")
	require.NoError(t, err)
	resp.Body.Close()

	body := map[string]string{}
	require.NoError(t, json.NewDecoder(resp.Body).Decode(&body)) // this should fail
	assert.Equal(t, map[string]string{"foo": "bar"}, body)
}