encoding/json: encode \t as \t instead of \u0009

Shorter and easier to read form for a common character.

LGTM=bradfitz
R=adg, bradfitz
CC=golang-codereviews, zimmski
https://golang.org/cl/162340043

src/encoding/json/encode.go

@@ -805,6 +805,9 @@ func (e *encodeState) string(s string) (int, error) {
 			case '\r':
 				e.WriteByte('\\')
 				e.WriteByte('r')
+			case '\t':
+				e.WriteByte('\\')
+				e.WriteByte('t')
 			default:
 				// This encodes bytes < 0x20 except for \n and \r,
 				// as well as <, > and &. The latter are escaped because they
@@ -878,9 +881,12 @@ func (e *encodeState) stringBytes(s []byte) (int, error) {
 			case '\r':
 				e.WriteByte('\\')
 				e.WriteByte('r')
+			case '\t':
+				e.WriteByte('\\')
+				e.WriteByte('t')
 			default:
 				// This encodes bytes < 0x20 except for \n and \r,
-				// as well as < and >. The latter are escaped because they
+				// as well as <, >, and &. The latter are escaped because they
 				// can lead to security holes when user-controlled strings
 				// are rendered into JSON and served to some browsers.
 				e.WriteString(`\u00`)

src/encoding/json/encode_test.go

@@ -478,3 +478,55 @@ func TestEncodePointerString(t *testing.T) {
 		t.Fatalf("*N = %d; want 42", *back.N)
 	}
 }
+
+var encodeStringTests = []struct {
+	in  string
+	out string
+}{
+	{"\x00", `"\u0000"`},
+	{"\x01", `"\u0001"`},
+	{"\x02", `"\u0002"`},
+	{"\x03", `"\u0003"`},
+	{"\x04", `"\u0004"`},
+	{"\x05", `"\u0005"`},
+	{"\x06", `"\u0006"`},
+	{"\x07", `"\u0007"`},
+	{"\x08", `"\u0008"`},
+	{"\x09", `"\t"`},
+	{"\x0a", `"\n"`},
+	{"\x0b", `"\u000b"`},
+	{"\x0c", `"\u000c"`},
+	{"\x0d", `"\r"`},
+	{"\x0e", `"\u000e"`},
+	{"\x0f", `"\u000f"`},
+	{"\x10", `"\u0010"`},
+	{"\x11", `"\u0011"`},
+	{"\x12", `"\u0012"`},
+	{"\x13", `"\u0013"`},
+	{"\x14", `"\u0014"`},
+	{"\x15", `"\u0015"`},
+	{"\x16", `"\u0016"`},
+	{"\x17", `"\u0017"`},
+	{"\x18", `"\u0018"`},
+	{"\x19", `"\u0019"`},
+	{"\x1a", `"\u001a"`},
+	{"\x1b", `"\u001b"`},
+	{"\x1c", `"\u001c"`},
+	{"\x1d", `"\u001d"`},
+	{"\x1e", `"\u001e"`},
+	{"\x1f", `"\u001f"`},
+}
+
+func TestEncodeString(t *testing.T) {
+	for _, tt := range encodeStringTests {
+		b, err := Marshal(tt.in)
+		if err != nil {
+			t.Errorf("Marshal(%q): %v", tt.in, err)
+			continue
+		}
+		out := string(b)
+		if out != tt.out {
+			t.Errorf("Marshal(%q) = %#q, want %#q", tt.in, out, tt.out)
+		}
+	}
+}