Improve stdlib compatibility

1. Null values for primitive types no longer clear the original value in
the destination object.

2. Dereference multiple levels of pointers in the destination interface{}
type before unmarshaling into it. This is needed to match stdlib
behavior when working with nested interface{} fields. If the destination
object is a pointer to interface{} then the incoming nil value should
nil out the destination object but keep the reference to that nil value
on its parent object. However if the destination object is an
interface{} value it should set the reference to nil but keep the
original object intact.

3. Correctly handle typed nil decode destinations.

jsoniter_interface_test.go

@@ -437,3 +437,124 @@ func Test_marshal_nil_nonempty_interface(t *testing.T) {
 	should.NoError(err)
 	should.Equal(nil, obj.Field)
 }
+
+func Test_overwrite_interface_ptr_value_with_nil(t *testing.T) {
+	type Wrapper struct {
+		Payload interface{} `json:"payload,omitempty"`
+	}
+	type Payload struct {
+		Value int `json:"val,omitempty"`
+	}
+
+	should := require.New(t)
+
+	payload := &Payload{}
+	wrapper := &Wrapper{
+		Payload: &payload,
+	}
+
+	err := json.Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(&payload, wrapper.Payload)
+	should.Equal(42, (*(wrapper.Payload.(**Payload))).Value)
+
+	err = json.Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(&payload, wrapper.Payload)
+	should.Equal((*Payload)(nil), payload)
+
+	payload = &Payload{}
+	wrapper = &Wrapper{
+		Payload: &payload,
+	}
+
+	err = Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(&payload, wrapper.Payload)
+	should.Equal(42, (*(wrapper.Payload.(**Payload))).Value)
+
+	err = Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(&payload, wrapper.Payload)
+	should.Equal((*Payload)(nil), payload)
+}
+
+func Test_overwrite_interface_value_with_nil(t *testing.T) {
+	type Wrapper struct {
+		Payload interface{} `json:"payload,omitempty"`
+	}
+	type Payload struct {
+		Value int `json:"val,omitempty"`
+	}
+
+	should := require.New(t)
+
+	payload := &Payload{}
+	wrapper := &Wrapper{
+		Payload: payload,
+	}
+
+	err := json.Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(42, (*(wrapper.Payload.(*Payload))).Value)
+
+	err = json.Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(nil, wrapper.Payload)
+	should.Equal(42, payload.Value)
+
+	payload = &Payload{}
+	wrapper = &Wrapper{
+		Payload: payload,
+	}
+
+	err = Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(42, (*(wrapper.Payload.(*Payload))).Value)
+
+	err = Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Equal(nil, err)
+	should.Equal(nil, wrapper.Payload)
+	should.Equal(42, payload.Value)
+}
+
+func Test_unmarshal_into_nil(t *testing.T) {
+	type Payload struct {
+		Value int `json:"val,omitempty"`
+	}
+	type Wrapper struct {
+		Payload interface{} `json:"payload,omitempty"`
+	}
+
+	should := require.New(t)
+
+	var payload *Payload
+	wrapper := &Wrapper{
+		Payload: payload,
+	}
+
+	err := json.Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Nil(err)
+	should.NotNil(wrapper.Payload)
+	should.Nil(payload)
+
+	err = json.Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Nil(err)
+	should.Nil(wrapper.Payload)
+	should.Nil(payload)
+
+	payload = nil
+	wrapper = &Wrapper{
+		Payload: payload,
+	}
+
+	err = Unmarshal([]byte(`{"payload": {"val": 42}}`), &wrapper)
+	should.Nil(err)
+	should.NotNil(wrapper.Payload)
+	should.Nil(payload)
+
+	err = Unmarshal([]byte(`{"payload": null}`), &wrapper)
+	should.Nil(err)
+	should.Nil(wrapper.Payload)
+	should.Nil(payload)
+}