mime: sort attributes in FormatMediaType

Map iteration order issue. Go 1.2 and earlier had stable results
for small maps.

Fixes #8115

LGTM=r, rsc
R=golang-codereviews, r
CC=dsymonds, golang-codereviews, iant, rsc
https://golang.org/cl/98580047

src/pkg/mime/mediatype.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"sort"
 	"strings"
 	"unicode"
 )
@@ -31,7 +32,14 @@ func FormatMediaType(t string, param map[string]string) string {
 	b.WriteByte('/')
 	b.WriteString(strings.ToLower(sub))
 
-	for attribute, value := range param {
+	attrs := make([]string, 0, len(param))
+	for a := range param {
+		attrs = append(attrs, a)
+	}
+	sort.Strings(attrs)
+
+	for _, attribute := range attrs {
+		value := param[attribute]
 		b.WriteByte(';')
 		b.WriteByte(' ')
 		if !isToken(attribute) {

src/pkg/mime/mediatype_test.go

@@ -293,6 +293,7 @@ var formatTests = []formatTest{
 	{"foo/BAR", map[string]string{"": "empty attribute"}, ""},
 	{"foo/BAR", map[string]string{"bad attribute": "baz"}, ""},
 	{"foo/BAR", map[string]string{"nonascii": "not an ascii character: ä"}, ""},
+	{"foo/bar", map[string]string{"a": "av", "b": "bv", "c": "cv"}, "foo/bar; a=av; b=bv; c=cv"},
 }
 
 func TestFormatMediaType(t *testing.T) {