xml: omit newline at beginning of MarshalIndent output

(Still valid XML.)

Fixes #3354.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7288047

src/pkg/encoding/xml/marshal.go

@@ -124,6 +124,7 @@ type printer struct {
 	prefix     string
 	depth      int
 	indentedIn bool
+	putNewline bool
 }
 
 // marshalValue writes one or more XML elements representing val.
@@ -394,7 +395,11 @@ func (p *printer) writeIndent(depthDelta int) {
 		}
 		p.indentedIn = false
 	}
-	p.WriteByte('\n')
+	if p.putNewline {
+		p.WriteByte('\n')
+	} else {
+		p.putNewline = true
+	}
 	if len(p.prefix) > 0 {
 		p.WriteString(p.prefix)
 	}

src/pkg/encoding/xml/marshal_test.go

@@ -7,6 +7,7 @@ package xml
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"reflect"
 	"strconv"
@@ -840,6 +841,24 @@ var marshalErrorTests = []struct {
 	},
 }
 
+var marshalIndentTests = []struct {
+	Value     interface{}
+	Prefix    string
+	Indent    string
+	ExpectXML string
+}{
+	{
+		Value: &SecretAgent{
+			Handle:    "007",
+			Identity:  "James Bond",
+			Obfuscate: "<redacted/>",
+		},
+		Prefix:    "",
+		Indent:    "\t",
+		ExpectXML: fmt.Sprintf("<agent handle=\"007\">\n\t<Identity>James Bond</Identity><redacted/>\n</agent>"),
+	},
+}
+
 func TestMarshalErrors(t *testing.T) {
 	for idx, test := range marshalErrorTests {
 		_, err := Marshal(test.Value)
@@ -884,6 +903,19 @@ func TestUnmarshal(t *testing.T) {
 	}
 }
 
+func TestMarshalIndent(t *testing.T) {
+	for i, test := range marshalIndentTests {
+		data, err := MarshalIndent(test.Value, test.Prefix, test.Indent)
+		if err != nil {
+			t.Errorf("#%d: Error: %s", i, err)
+			continue
+		}
+		if got, want := string(data), test.ExpectXML; got != want {
+			t.Errorf("#%d: MarshalIndent:\nGot:%s\nWant:\n%s", i, got, want)
+		}
+	}
+}
+
 type limitedBytesWriter struct {
 	w      io.Writer
 	remain int // until writes fail