Fix linter issues for golangci-lint 1.54.2

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

content/helpers_test.go

@@ -143,6 +143,7 @@ func TestCopy(t *testing.T) {
 	}
 
 	for _, testcase := range testcases {
+		testcase := testcase
 		t.Run(testcase.name, func(t *testing.T) {
 			err := Copy(context.Background(),
 				&testcase.writer,