Allow null booleans

Make sure we do the same thing as stdlib with null booleans by not
touching the original value and discarding the null.

Another somewhat related change is nulling out null interface values in
the original structure. This also matches stdlib behavior.

jsoniter_interface_test.go

@@ -3,9 +3,10 @@ package jsoniter
 import (
 	"encoding/json"
 	"fmt"
-	"github.com/stretchr/testify/require"
 	"testing"
 	"unsafe"
+
+	"github.com/stretchr/testify/require"
 )
 
 func Test_write_array_of_interface(t *testing.T) {
@@ -313,3 +314,40 @@ func Test_unmarshal_ptr_to_interface(t *testing.T) {
 	should.Nil(err)
 	should.Equal("&{value}", fmt.Sprintf("%v", obj))
 }
+
+func Test_nil_out_null_interface(t *testing.T) {
+	type TestData struct {
+		Field interface{} `json:"field"`
+	}
+	should := require.New(t)
+
+	var boolVar bool
+	obj := TestData{
+		Field: &boolVar,
+	}
+
+	data1 := []byte(`{"field": true}`)
+
+	err := Unmarshal(data1, &obj)
+	should.Equal(nil, err)
+	should.Equal(true, *(obj.Field.(*bool)))
+
+	data2 := []byte(`{"field": null}`)
+
+	err = Unmarshal(data2, &obj)
+	should.Equal(nil, err)
+	should.Equal(nil, obj.Field)
+
+	// Checking stdlib behavior matches.
+	obj2 := TestData{
+		Field: &boolVar,
+	}
+
+	err = json.Unmarshal(data1, &obj2)
+	should.Equal(nil, err)
+	should.Equal(true, *(obj2.Field.(*bool)))
+
+	err = json.Unmarshal(data2, &obj2)
+	should.Equal(nil, err)
+	should.Equal(nil, obj2.Field)
+}