Run tests against Go 1.10 and fix a bug exposed by doing so

.gitlab-ci.yml

@@ -13,6 +13,10 @@ test using go 1.9:
   image: golang:1.9
   <<: *test_definition
 
+test using go 1.10:
+  image: golang:1.10
+  <<: *test_definition
+
 test:release:
   only:
     - tags

internal/helper/writeafterreader_test.go

@@ -85,7 +85,7 @@ func TestComplainingWriterSanity(t *testing.T) {
 
 	w.CheerUp()
 	if _, err := io.Copy(w, testReader(testData)); err != nil {
-		t.Error("copy after CheerUp: %v", err)
+		t.Errorf("copy after CheerUp: %v", err)
 	}
 
 	if result := recorder.String(); result != testData {