Fix route_add{,_test}.go to use an io.Writer for saving the response

Also add a TODO regarding untested response payload.

Co-authored-by: Héctor Hurtado <hector.hurtado@bbva.com>

internal/client/route_add_test.go

@@ -21,7 +21,10 @@ func TestSuccessOnCorrectRoute(t *testing.T) {
 		Reply(http.StatusCreated).
 		JSON(map[string]string{})
 
-	err := AddRoute("http://localhost", "/hello", "GET", "", "echo Hello World | kapow set /response/body")
+	// TODO: As per the spec¹, the call should return a JSON body with the info of the new route
+	// We should consider this in the mocked server and in the test
+	// ¹: https://github.com/BBVA/kapow/tree/master/spec#insert-a-route
+	err := AddRoute("http://localhost", "/hello", "GET", "", "echo Hello World | kapow set /response/body", nil)
 	if err != nil {
 		t.Errorf("Unexpected error: %s", err)
 	}