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/example_test.go

@@ -33,6 +33,29 @@ func ExampleMarshal() {
 	// {"ID":1,"Name":"Reds","Colors":["Crimson","Red","Ruby","Maroon"]}
 }
 
+func ExampleMarshal_overflow() {
+	type Record struct {
+		ID   string
+		Seq  int
+		Meta map[string]string `json:",overflow"`
+	}
+	r := Record{
+		ID:  "CheeseWhiz",
+		Seq: 42,
+		Meta: map[string]string{
+			"Created":   "1980-06-20",
+			"Destroyed": "1998-02-06",
+		},
+	}
+	b, err := json.Marshal(r)
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	os.Stdout.Write(b)
+	// Output:
+	// {"ID":"CheeseWhiz","Seq":42,"Created":"1980-06-20","Destroyed":"1998-02-06"}
+}
+
 func ExampleUnmarshal() {
 	var jsonBlob = []byte(`[
 		{"Name": "Platypus", "Order": "Monotremata"},
@@ -52,6 +75,29 @@ func ExampleUnmarshal() {
 	// [{Name:Platypus Order:Monotremata} {Name:Quoll Order:Dasyuromorphia}]
 }
 
+func ExampleUnmarshal_overflow() {
+	var jsonBlob = []byte(`
+		{
+			"Token": "Kaip4uM1ieng6Eiw",
+			"User":  "bimmler",
+			"Animal": "rabbit"
+		}
+	`)
+	type Auth struct {
+		Token string
+		User  string
+		Extra map[string]string `json:",overflow"`
+	}
+	var auth Auth
+	err := json.Unmarshal(jsonBlob, &auth)
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	fmt.Printf("%+v", auth)
+	// Output:
+	// {Token:Kaip4uM1ieng6Eiw User:bimmler Extra:map[Animal:rabbit]}
+}
+
 // This example uses a Decoder to decode a stream of distinct JSON values.
 func ExampleDecoder() {
 	const jsonStream = `