the Gerrit change.

**Please ensure you adhere to every item in this list.**

More info can be found at https://github.com/golang/go/wiki/CommitMessage

+ The PR title is formatted as follows: `net/http: frob the quux before blarfing`
  + The package name goes before the colon
  + The part after the colon uses the verb tense + phrase that completes the blank in,
    "This change modifies Go to ___________"
  + Lowercase verb after the colon

	if err != nil {
		t.Fatalf("os.Stat: %v", err)
	}
	contents := strings.Repeat(" ", int(hdr.Size))

	hdr.Xattrs = map[string]string{
		"foo": "foo",
		"bar": "bar",
		"baz": "baz",
		"qux": "qux",
	}

	var buf bytes.Buffer
	writer := NewWriter(&buf)
	if err := writer.WriteHeader(hdr); err != nil {
		t.Fatal(err)
	}
	if _, err = writer.Write([]byte(contents)); err != nil {
		t.Fatal(err)