encoding/json: add "overflow" struct tag option

Fixes #6213.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://golang.org/cl/13180043

src/pkg/encoding/json/decode_test.go

@@ -1275,3 +1275,23 @@ func TestSkipArrayObjects(t *testing.T) {
 		t.Errorf("got error %q, want nil", err)
 	}
 }
+
+func TestDecodeOverflow(t *testing.T) {
+	json := `{"A":1,"B":2,"C":3}`
+	type S struct {
+		A int
+		E map[string]interface{} `json:",overflow"`
+		C int
+	}
+	var (
+		want = S{1, map[string]interface{}{"B": float64(2)}, 3}
+		dest S
+	)
+	err := Unmarshal([]byte(json), &dest)
+	if err != nil {
+		t.Errorf("got error %q, want nil", err)
+	}
+	if !reflect.DeepEqual(dest, want) {
+		t.Errorf("Got %+v; want %+v", dest, want)
+	}
+}