encoding/json: escape & always

There are a few different places in the code that escape
possibly-problematic characters like < > and &.
This one was the only one missing &, so add it.

This means that if you Marshal a string, you get the
same answer you do if you Marshal a string and
pass it through the compactor. (Ironically, the
compaction makes the string longer.)

Because html/template invokes json.Marshal to
prepare escaped strings for JavaScript, this changes
the form of some of the escaped strings, but not
their meaning.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12708044

src/pkg/encoding/json/encode.go

@@ -734,7 +734,7 @@ func (e *encodeState) string(s string) (int, error) {
 	start := 0
 	for i := 0; i < len(s); {
 		if b := s[i]; b < utf8.RuneSelf {
-			if 0x20 <= b && b != '\\' && b != '"' && b != '<' && b != '>' {
+			if 0x20 <= b && b != '\\' && b != '"' && b != '<' && b != '>' && b != '&' {
 				i++
 				continue
 			}

src/pkg/html/template/content_test.go

@@ -123,29 +123,29 @@ func TestTypedContent(t *testing.T) {
 		{
 			`<script>alert({{.}})</script>`,
 			[]string{
-				`"\u003cb\u003e \"foo%\" O'Reilly &bar;"`,
+				`"\u003cb\u003e \"foo%\" O'Reilly \u0026bar;"`,
 				`"a[href =~ \"//example.com\"]#foo"`,
-				`"Hello, \u003cb\u003eWorld\u003c/b\u003e &amp;tc!"`,
+				`"Hello, \u003cb\u003eWorld\u003c/b\u003e \u0026amp;tc!"`,
 				`" dir=\"ltr\""`,
 				// Not escaped.
 				`c && alert("Hello, World!");`,
 				// Escape sequence not over-escaped.
 				`"Hello, World & O'Reilly\x21"`,
-				`"greeting=H%69&addressee=(World)"`,
+				`"greeting=H%69\u0026addressee=(World)"`,
 			},
 		},
 		{
 			`<button onclick="alert({{.}})">`,
 			[]string{
-				`&#34;\u003cb\u003e \&#34;foo%\&#34; O&#39;Reilly &amp;bar;&#34;`,
+				`&#34;\u003cb\u003e \&#34;foo%\&#34; O&#39;Reilly \u0026bar;&#34;`,
 				`&#34;a[href =~ \&#34;//example.com\&#34;]#foo&#34;`,
-				`&#34;Hello, \u003cb\u003eWorld\u003c/b\u003e &amp;amp;tc!&#34;`,
+				`&#34;Hello, \u003cb\u003eWorld\u003c/b\u003e \u0026amp;tc!&#34;`,
 				`&#34; dir=\&#34;ltr\&#34;&#34;`,
 				// Not JS escaped but HTML escaped.
 				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
 				// Escape sequence not over-escaped.
 				`&#34;Hello, World &amp; O&#39;Reilly\x21&#34;`,
-				`&#34;greeting=H%69&amp;addressee=(World)&#34;`,
+				`&#34;greeting=H%69\u0026addressee=(World)&#34;`,
 			},
 		},
 		{

src/pkg/html/template/escape_test.go

@@ -538,7 +538,7 @@ func TestEscape(t *testing.T) {
 		{
 			"typed HTML in script",
 			`<button onclick="alert({{.W}})">`,
-			`<button onclick="alert(&#34;&amp;iexcl;\u003cb class=\&#34;foo\&#34;\u003eHello\u003c/b\u003e, \u003ctextarea\u003eO&#39;World\u003c/textarea\u003e!&#34;)">`,
+			`<button onclick="alert(&#34;\u0026iexcl;\u003cb class=\&#34;foo\&#34;\u003eHello\u003c/b\u003e, \u003ctextarea\u003eO&#39;World\u003c/textarea\u003e!&#34;)">`,
 		},
 		{
 			"typed HTML in RCDATA",