encoding/json: make ,string work when encoding pointer fields

It was respected by unmarshal, but not marshal, so they didn't
round-trip.

Fixes #8582

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/132960043

src/pkg/encoding/json/encode.go

@@ -696,12 +696,12 @@ type ptrEncoder struct {
 	elemEnc encoderFunc
 }
 
-func (pe *ptrEncoder) encode(e *encodeState, v reflect.Value, _ bool) {
+func (pe *ptrEncoder) encode(e *encodeState, v reflect.Value, quoted bool) {
 	if v.IsNil() {
 		e.WriteString("null")
 		return
 	}
-	pe.elemEnc(e, v.Elem(), false)
+	pe.elemEnc(e, v.Elem(), quoted)
 }
 
 func newPtrEncoder(t reflect.Type) encoderFunc {

src/pkg/encoding/json/encode_test.go

@@ -452,3 +452,29 @@ func TestHTMLEscape(t *testing.T) {
 		t.Errorf("HTMLEscape(&b, []byte(m)) = %s; want %s", b.Bytes(), want.Bytes())
 	}
 }
+
+// golang.org/issue/8582
+func TestEncodePointerString(t *testing.T) {
+	type stringPointer struct {
+		N *int64 `json:"n,string"`
+	}
+	var n int64 = 42
+	b, err := Marshal(stringPointer{N: &n})
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got, want := string(b), `{"n":"42"}`; got != want {
+		t.Errorf("Marshal = %s, want %s", got, want)
+	}
+	var back stringPointer
+	err = Unmarshal(b, &back)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if back.N == nil {
+		t.Fatalf("Unmarshalled nil N field")
+	}
+	if *back.N != 42 {
+		t.Fatalf("*N = %d; want 42", *back.N)
+	}
+}