code: func Payload(want []byte) TransportChecker {
  	return func(t *testing.T, h header.Transport) {
  		if got := h.Payload(); !reflect.DeepEqual(got, want) {
  			t.Errorf("Wrong payload, got %v, want %v", got, want)
  		}
  	}
  }
docs: // Payload creates a checker that checks the payload.