func makeReqCancel(req *http.Request) func(http.RoundTripper) {
	return func(r http.RoundTripper) {
		if t, ok := r.(*http.Transport); ok {
			t.CancelRequest(req)
		}
	}
}
makeReqCancel returns a closure that cancels the given http.Request
when called.
package transport
import "net/http"