Use http.Error to also set text/plain content type

Fixes #31

handler.go

@@ -60,7 +60,7 @@ type CSRFHandler struct {
 }
 
 func defaultFailureHandler(w http.ResponseWriter, r *http.Request) {
-	w.WriteHeader(FailureCode)
+	http.Error(w, "", FailureCode)
 }
 
 // Extracts the "sent" token from the request

handler_test.go

@@ -249,6 +249,13 @@ func TestNoTokenFails(t *testing.T) {
 		t.Errorf("The check should've failed with the code %d, but instead, it"+
 			" returned code %d", FailureCode, writer.Code)
 	}
+
+	expectedContentType := "text/plain; charset=utf-8"
+	actualContentType := writer.Header().Get("Content-Type")
+	if actualContentType != expectedContentType {
+		t.Errorf("The check should've failed with content type %s, but instead, it"+
+			" returned content type %s", expectedContentType, actualContentType)
+	}
 }
 
 func TestWrongTokenFails(t *testing.T) {
@@ -274,6 +281,13 @@ func TestWrongTokenFails(t *testing.T) {
 		t.Errorf("The check should've failed with the code %d, but instead, it"+
 			" returned code %d", FailureCode, writer.Code)
 	}
+
+	expectedContentType := "text/plain; charset=utf-8"
+	actualContentType := writer.Header().Get("Content-Type")
+	if actualContentType != expectedContentType {
+		t.Errorf("The check should've failed with content type %s, but instead, it"+
+			" returned content type %s", expectedContentType, actualContentType)
+	}
 }
 
 // For this and similar tests we start a test server

testutils_test.go

@@ -40,7 +40,7 @@ func correctReason(t *testing.T, reason error) http.Handler {
 				" but it failed with the reason %#v", reason, got)
 		}
 		// Writes the default failure code
-		w.WriteHeader(FailureCode)
+		http.Error(w, "", FailureCode)
 	}
 
 	return http.HandlerFunc(fn)