encoding/json: add Decoder.Buffered accessor to get overread data

Otherwise it's impossible to know how much data from the
json.Decoder's underlying Reader was actually consumed.

The old fix from golang.org/issue/1955 just added docs. This
provides an actual mechanism.

Update #1955

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/7181053

src/pkg/encoding/json/stream_test.go

@@ -6,8 +6,10 @@ package json
 
 import (
 	"bytes"
+	"io/ioutil"
 	"net"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -83,6 +85,28 @@ func TestDecoder(t *testing.T) {
 	}
 }
 
+func TestDecoderBuffered(t *testing.T) {
+	r := strings.NewReader(`{"Name": "Gopher"} extra `)
+	var m struct {
+		Name string
+	}
+	d := NewDecoder(r)
+	err := d.Decode(&m)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m.Name != "Gopher" {
+		t.Errorf("Name = %q; want Gopher", m.Name)
+	}
+	rest, err := ioutil.ReadAll(d.Buffered())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := string(rest), " extra "; g != w {
+		t.Errorf("Remaining = %q; want %q", g, w)
+	}
+}
+
 func nlines(s string, n int) string {
 	if n <= 0 {
 		return ""